Why was Data never promoted?

[Shamrock Holmes]

Quite possibly so. Indirect dating puts the O'Brien event at 2347, while our first solid proof of something other than the (now collarless) Maroons comes from the photo in "Suddenly Human", dated at 2353.

It's a bit funny, though, that O'Brien would equate the Maroons Gold with "the gold suit" when the Gold he is wearing at the time (of "Paradise") signifies something else altogether, something perhaps closer to the Maroons Grey.

Maroons Grey was Navigation (possibly Weapons/Armory), Communications and Space Sciences, Maroons Gold was Engineering and Helm. IMO, despite not being a big fan of the uniform in general, I favour the "Engineering - Comms/Helm/Nav (potentially inc Command) - Security & Services - Science - Medical" split from TMP.

Perhaps this "infantry" idea is off base altogether, and even any sort of a security role is a misunderstanding, with O'Brien in 2347 beaming down "with a squad" rather than as part of a squad. But nothing indicates he would have been a medic or a communications specialist or anything like that. Perhaps he was an engineer already, or at least a tech, and his fumbling with the field transporter just tells us his technical specialty was something else?

Unlikely,

OBRIEN: What is that supposed to mean?
GARAK: Oh, we all know your distinguished war record. How you led two dozen men against the Barrica encampment and took out an entire regiment of Cardassians. If you play Kotra with half that brazenness, we'd have quite a match.
OBRIEN: I'm not a soldier anymore. I'm an engineer.
GARAK: I see. So when you and Doctor Bashir go into the holosuites for hours at a time you're just repairing them?
OBRIEN: What's your point, Garak?

OBRIEN: Because he wants to get under my skin.
NOG: But you were a hero, weren't you?
OBRIEN: That doesn't mean I like thinking about what happened then. I was a soldier, Nog. Sometimes soldiers have to kill.
GARAK: Come now, Chief, don't be so modest. You did a lot of killing.
NOG: How'd you get in here? Both doors are secure.

We have reasonable evidence that a separate divison of soldiers existed during the period identified as soldiers by Kirk (Federation forces doesn't appear to be their full proper name), and O'Brien identifies himself as a soldier during the period.

We also have this:

OBRIEN: No, it's true. It wasn't till I got to the Cardassian front I found I had talents I never knew I had.
SISKO: At the front?
OBRIEN: It was a matter of figuring out how to get a field transporter operational in ten minutes or wind up a Cardassian prisoner of war. Now, I didn't know a transporter from a turbolift in those days but somehow, in nine minutes fifty three seconds, I got that thing to work. I got thirteen men safely off the surface of Setlik Three. Next thing I know I'm the tactical officer on the Rutledge. That's how I got the gold suit.
SISKO: Well, Jake's ready for a new suit too. The boy seems to grow a centimetre each week. You know, he struck me out with a curve ball the other day. First time.
OBRIEN: They grow up in a hurry, don't they, sir.

So he wasn't wearing gold (worn by engineers, pilots and possibly Armory during the period), until that point, therefore while we can't say that he wasn't Security (wearing Dark Green) rather than "ground forces" I think it's probable.

 

[SpyOne]

Or then it means that TOS Gold gives the full Line Officer clearance, TOS Blue is Staff Officer with all sorts of limitations, and TOS Red sits somewhere in between, perhaps mainly applying to specialists who won't get to command much even if they might be entitled to.

Navy people I talked to suggested Limited Duty Officers, which is almost exactly what you described.
You stay within one department (like Engineering) because that's your specialty. As a result, you rise in rank slower.
Meanwhile Command guys (Line Officers) keep getting moved to different departments so they have as broad an experience as possible.

So, Worf took over from Tasha Yar, too. It didn't appear as if Yar had been his boss, or that this would in some way be a requirement for taking over a position. People just get reshuffled. And perhaps department heads get reshuffled before underlings get promoted to bosses?

Well, in an awful lot of first season episodes it does appear that Worf works for Yar. Like whenever she's asked to assemble a Security team she's like "Okay, Worf and .... Ensign Ricky".
Still, it appears I misremembered that, as the actual line is this is the second time Worf "replaced a crewmate that died."

Because nobody could temp LaForge at Engineering, certainly not Worf?

I agree that Worf is probably not going to be put in charge of Engineering, but part of why I agree is that Geordi almost certainly has a Lieutenant of equal rank to Worf with more experience serving under him.
At a reasonable guess, at least three: one for each shift. They actually run Engineering while Geordi runs them.

But yes, we can't prove that with on screen evidence. And enough people appear in only one episode that, even if we can establish several such Lieutenants existed previously, we can't be sure they were aboard then.
It just seems odd in retrospect, that a guy who only just made full Lieutenant took over the job of one of the most senior Lieutenant Commanders.

I, too, value your perspective since it differs from mine. You challenge my assumptions and help me see where I've mistaken my own opinion for established fact.

 

[Shamrock Holmes]

I agree that Worf is probably not going to be put in charge of Engineering, but part of why I agree is that Geordi almost certainly has a Lieutenant of equal rank to Worf with more experience serving under him.
At a reasonable guess, at least three: one for each shift. They actually run Engineering while Geordi runs them.

As of Hollow Pursuits there are at least three Engineering Officers that work closely with LaForge, Lieutenant JGs Duffy, Costa (maybe), Myers and Barclay. Duffy appears to have replaced Lt Cmdr Argyle as the M/ARA specialist, Costa does both reactors and data systems (he's one of their best programmers), Barclay does systems, Myers appears to work on the engines/power systems.

But yes, we can't prove that with on screen evidence. And enough people appear in only one episode that, even if we can establish several such Lieutenants existed previously, we can't be sure they were aboard then.
It just seems odd in retrospect, that a guy who only just made full Lieutenant took over the job of one of the most senior Lieutenant Commanders.

Actually, it's possible but not definative that he jumped over three Lt Cmdrs (Argyle, MacDougal and Lynch.

 

[Grendelsbayne]

Season 1 TNG claimed the Enterprise had multiple 'chief engineers'. This doesn't seem to be the case later on in the show(?), but it's unclear when the change happened. Perhaps Geordi's promotion to 'chief engineer' was not, at first, actually the position of chief 'chief engineer', but rather junior 'chief engineer'. Then the others are transfered out, whatever, eventually leaving him fully in charge.

 

[Shamrock Holmes]

Season 1 TNG claimed the Enterprise had multiple 'chief engineers'. This doesn't seem to be the case later on in the show(?), but it's unclear when the change happened.

No references to additional Chief Engineers survived LaForge's promotion to ChEng (Riker's role as overall supervisor for propulsion systems (per WNOHGB) disappears around the same time.

Perhaps Geordi's promotion to 'chief engineer' was not, at first, actually the position of chief 'chief engineer', but rather junior 'chief engineer'. Then the others are transfered out, whatever, eventually leaving him fully in charge.

All three "Co-Chiefs" are mentioned (in the error?) in the remastered version of Galaxy's Child in Season 4, but are never seen "on screen" after Season 1.

 

[Timo]

Unlikely [..]O'Brien identifies himself as a soldier during the period.

Ah, but that's just it - "during the period" is limited to 2362. O'Brien's reputation as a soldier comes exclusively from that period, and is irrelevant to the issue of his profession in 2347.

Or, rather, not. After all, 2347 marked a transition for him. So since he is a soldier in 2362, it directly follows he was not one yet when he beamed down with Maxwell and saved his bacon with the field transporter repair.

Season 1 TNG claimed the Enterprise had multiple 'chief engineers'.

And make no mistake, it actually directly claimed this, rather than merely showing us multiple chief engineers.

Might be the E-D was a training ship for Galaxy class CEOs. Once that stage of the ship's mission was over, Picard was free to further shuffle his team, and the retroactive backstory from "The Next Phase" is that LaForge had always been an engineer-in-waiting.

But other options certainly exist, and it may simply be that the comment about "one of our Chief Engineers" was merely sarcasm about the fact that these people didn't last long.

Timo Saloniemi

 

[Shamrock Holmes]

Ah, but that's just it - "during the period" is limited to 2362. O'Brien's reputation as a soldier comes exclusively from that period, and is irrelevant to the issue of his profession in 2347.

Or, rather, not. After all, 2347 marked a transition for him. So since he is a soldier in 2362, it directly follows he was not one yet when he beamed down with Maxwell and saved his bacon with the field transporter repair.

There's a bit more dialogue before the quoted portion that might help:

OBRIEN: I'm sure, Commander. Jake's a good fellow. I'll enjoy working with him.
SISKO: I don't want you to make it easy for him, Chief. He has to know what it takes to make it in Starfleet.
OBRIEN: I'll have him up to his elbows in thorium grease, sir.
SISKO: It's not going to be easy for him. He placed in the lower third of his age group in mechanical aptitude.
OBRIEN: Good for him. So did I.
SISKO: You? Come on.
OBRIEN: No, it's true. It wasn't till I got to the Cardassian front I found I had talents I never knew I had.
SISKO: At the front?


Which suggests that while O'Brien had some theoritical knowledge prior to this point (from building model starships?), Setlik III was his first taste of practical engineering and indeed he may be better with Cardassian tech than he is Starfleet, accounting for part of the reason he got the "pleasure" of the DS9 assignment.

FTW, the 2347 reference is a largely off the cuff remark to Barclay, rather than the more plot relevant references to the Setlik III incident taking place in 2362.

 

[Timo]

So we eliminate a few options more. What would be our best bet for the Starfleet sub-profession that O'Brien chose in order to escape his cello lessons? Something that probably takes less time to attain than most (not just a likely criterion for Miles, but something suggested by him being out in the field already in 2347 when he's but nineteen), doesn't require mechanical aptitude, isn't in the same branch as Tactical Officer...

Timo Saloniemi

 

[Shamrock Holmes]

Pretty much comes back to Security or Infantry. It's just about possible he was a pilot, but LaForge, Ro and Paris's displayed skills and knowledge at minimum suggest that there are engineering aspects to this role as well.