A lowly Lieutenant or Lieutenant jg would have sufficed.
Durst didn't want the job.
It was beneath him.
Department Head is not doing the job.
They can all do the job, it's not that hard.
Department head is Management.
Maybe Harry's Department is 5 people?
Harry makes the roster, polices the roster, and then writes the crew evaluations, promoting, demoting or transferring members of his team pursuant to their ability.
That's a few hours of tedious paperwork every week, which is a lot of work for an ensign, a walk in the park for a lieutenant JG and bollocks a Lt commander can put to bed in their sleep.
We ever see Harry do paperwork or manage anyone?
This?
Also a lot of what Dee and Riker said in "TNG Lower Decks" barely explaining how quarterly crew evaluations work.
Or do you think that Harry is always on the bridge 24/7?
Or that if he's not on the bridge, because he is Holodecking or sleeping, then no one is in staffing Operations?
Is Harry one of the thirteen?
Who else do we see at Ops?
Eh, I can't fault him for that. She took him by surprise. Not all guys got game. 
* "EYE OF THE NEEDLE" - Casually (indirectly) suggests they risk polluting the timeline.
* "REVULSION" - Doesn't step up to the plate with Seven of Nine.
* "SCIENTIFIC METHOD" - Recommends boosting the shields instead of increasing the ship's velocity.
* "TIMELESS" - Destroys Voyager in another timeline.
* "THE DISEASE" - Sleeps with an alien from a xenophobic race.
* "NIGHTINGALE" - Becomes slightly power hungry, in addition to getting duped.
No wonder Janeway wasn't keen on promoting Harry Kim.
* "REVULSION" - Doesn't step up to the plate with Seven of Nine.
* "SCIENTIFIC METHOD" - Recommends boosting the shields instead of increasing the ship's velocity.
* "TIMELESS" - Destroys Voyager in another timeline.
* "THE DISEASE" - Sleeps with an alien from a xenophobic race.
* "NIGHTINGALE" - Becomes slightly power hungry, in addition to getting duped.
No wonder Janeway wasn't keen on promoting Harry Kim.
Despite the severity of his actions, Janeway recognized the sincerity in Paris' words; that probably softened the punishment a bit.
As for his accomplishments?
* Piloting Voyager.
* Functioning as the ship's nurse.
* Fulfilling the role of commando.
* This one is slightly retroactive relative to "THIRTY DAYS", but, engaging in part-time engineering to the point where he successfully spearheaded the Delta Flyer project (and, not once, but twice).
Tom was a regular Renaissance Man. I will give Harry this much, though: the man can execute a mean combat roll.
Eh, I can't fault him for that. She took him by surprise. Not all guys got game.
You know she spooked him on purpose?
He had a laborious plan to kiss her on the cheek inside of three weeks, that Seven could see him grinding away at with her eyes closed. She was not trading peace of mind for a notch on his bed post, Seven figured that if she gave him too much woman, way too quickly, that he would blink and then she wouldn't have to keep over hearing his Walter Mitty plans to take her bicycle riding in 17th century Italy until she really began to appreciate how special he was, or anything else so lame you'd expect to see it on an early episode of Happy Days.
In "Good Shepherd", Seven calls him on his not leaving his staff enough to do during the late watch. So yes, he has underlings.
I have to disagree about "THE DISEASE". He didn't start getting the sex glow until after he hooked up with her, which means he was knowlingly defying protocols AND making a tense diplomatic situation more difficult by getting with Tal to begin with. So he was not under an alien influence... just his hormones.
Tom ends up with an alien species too, and Kim and Janeway are both flirting with aliens in Prime Factors.
It's a random rule used poorly.
What's funny is that this is one of those topics that never roused my interest (even prior to my encountering message boards, when I was simply watching the show)...until it kept appearing over and over in the most unlikely of places; now, I am glad the showrunners did what they did.
Harry staying an ensign makes no sense.
The eternal ensign was always a topic on this board that would come up from time to time but until I joined here I never gave it much thought either. Kind of in the same vein as having the entire TOS movie crew being commanders and captains. It just kind of is what it is and wasn't crucial to the story being told.
Since the inception of modern social media and podcasts the eternal ensign has taken on new life though. It drives clicks when brought up now and that's how they make money. Harry should have been promoted at some point, I do agree with that, especially in light of Tom's reinstatement, demotion, and then promotion.
And from a neutral perspective, of someone who knew the Trek universe and how promotions were managed therein, that's just obvious. But not only did it not happen, but...
1. The showrunners rubbed the viewer's nose in it, in Unimatrix Zero and Nightingale.
2. Even though Garrett Wang was quite available, the injustice was never corrected when Trek re-emerged.
