I don't think that complaining about how ethnicity was handled on TOS is a valid arguement. Folks tend to forget that ethnicity was handled very roughly on most all 1960's era tv shows. Trek was the first to show people of other cultures, races and genders working together in harmony as respected professionals and getting the job done!
Excluding all the other shows on before or at the same time as Star Trek that is: I Spy, Mission Impossible, Hogan's Heroes...
Don't forget: The Bill Cosby Show (yes he had one in 1969), Julia (the main character was black)
The statement that Star Trek was the first show to show people of multiple ethnicities working together is harminy is a false one. Hell, if you look at the ORIGINAL pilot
The Cage the crew is a carbon copy of the C57-D's crew of Forbidden Planet (all WASP)
The fact is - the ethnicity was added because of a memo that NBC execs sent out to production staff asking that more ethnicities be included in show casts to more mirror society, and that WHY you saw bill Cosby as a main character in the
I Spy TV series which premired on NBC in 1965 (the same year TOS was originally pitched to the network - although as we all know it wasn't picked up and pit on the schedule until 1966), and the memo was one of the issues brought up when NBC commisssioned the second pilot.)
As for whether 'Scotty' was necessary; honestly, he was a background character (and many shows of teh era had these -
Voyage To The Bottom of The Sea had a number of reccuring caracters like Chip Morton as the Exec; and crewman Kowoski.) ALL of the cast who became known as the 'Four' were that way - and had Star Trek not attained a cult following, I doubt these four would have played up their involvement that much.
Honestly, the show would have gotten along fine without ANY of them as it LOST a couple of reccuring cast members after the first season (Yeoman Rand and Lt. Riley) <---- No, do people remember them? Yes, but not as much as the other four background characters because whe nthe series first started getting a following, Grace Whitney was having real life issues (IE unable to join the starting convention circut and play her rolse up; and the actor behind Riley didn't do much at the start of the Star Trek convention circut either - BUT BOTH these characters had major plolines and a story devoted to them in season one (Rand's would have been
Dagger of the Mind if she hadn't been written out and Riley's was both
The Naked Time and
The Conscience of the King.
That why when the 'Gang of Four' take pot shots at William Shatner; and start to claim how the show wouldn't have survived as well over the years; or that Shatner 'treated them poorly' - I laugh as they WERE ALL character actors and were working in hollywood and KNEW the score and how 1960ies TV shows ran. William Shatmer WAS the headliner and star of the show. Spock was his sidekick AND (because he was such a talented character actor) DeForest Kelly managed to get a headline credit as well starting in Season 2.
The rest were all window dressing and interchangeable. Want proof? Does anyone think less of the Star Trek episode
The Doomsday Machine because Uhura isn't at teh Comm station - and instead we have a 'one off' of Lt. Palmer (Elizabeeth Rogers)? And yes, this IS a story where the background Engineer Scotty is needed, but again not unusual for 1960ies TV. Sulu is at the helm; BUT no Chekov either (but againb do you really notice and does it detract from the episode as a whole?
And remember; George Takei (Lt. Sulu) to the last half of the second season and a good portion of the third season OFF from Star Trek to go play a vietnamise character in the John Wayne film
The Green Berets -(which WAS a great carrer move at the time for George Takei to be sure); but again, given this I laugh when George Takei goes into 'how special' Star Trek was for him - yep, so special he took MAJOR time off to go do a feature film; whioch required Star Trek scripts to be rewritten and Chekov given Sulu's spot in those stories when
The Green Berets production took longer than expected.
So yeah, honestly, Star Treek would have done fine and IMO have been JUST AS popular as long as Kirk, Spock and McCoy were there. The rest opf the background characters could have been swapped and yes, it would have been different, but wouldn't have been affected much. (IMO)