I think the main problem with a utopian star trek is it just wouldn;t have got on air, as it was episodes showing the occupation of romulus after its defeat in the earth-romulan war were cut to ribbons by the censors.
I think they cut too close to the US's occupation of Hanoi and the Warzsaw pact and NATO forces in beijing ''restoring order'' in different part of china after the sino-soviet war.
If you can find the banned secnes from one of the romulus episodes whose name escapes me, the fanatical tal-shia resistance is pretty much a carbon copy of the red guards guerillas, and that image of riker head in hands sanctioning the use of orbital weapons on romulan villages is frankly haunting.
OOC: AFAIK OTL canon has a WWIII thrown in somewhere.
Like many I feel the main obstacle is the lack of historical realism. In the mirror universe there is a full scale nuclear war, from the ashes of which the federation rises, this just doesn;t seem realistic. The series of limited nuclear exchanges and smaller wars that dominated the last 50 years just mean this whole scenario is ASB.
Star Trek was and hopefully still is all about real human beings but in terrible situations. Not comic book book upstanding citizens as shown in the mirrror universe or the cackling maniacal evil do-ers all to popular in much western science fiction
As other posters have said the main problem it would completely mess with the story arc since it would change the q continuums judgement that humanity is unworthy to survive culminating in the famously dark final episode.
I also wonder what sort of movies a utopian Trek-verse would produce? I mean, Star Trek: The Motion Picture pushed the limits of what contemporary audiences were willing to tolerate, in terms of showing such wretchedness throughout the film, coupled with the shockingly bleak ending. And the movie series kept going downhill from there...
I think the sheer bleakness of the star trek movie franchise meant such movies only appealed to a dedicated trekkie fanbase. And your right they only got worse, wrath of khan where the formerly imprisoned genetically engineered philospher attempts to destroy the federations genesis weapons project was at least watchable, and you do get to see a brief spark of humanity in kirk as he openly breaks down weeping at spocks death. An element explored in later films like search for spock and generations in which kirk ultimately fails to find any redemption for the hcoices he has made in life. I guess generations picked up in terms of the effect of a positve utopian element like the nexus on picard, a storyline used to great effect when chronicling picards dissilusionment with his role and later dissertion but too many of the later movies like inssurection and nemesis returned to the endless negativity of the earlier movies, and they weren;t particularly well done either. The less said about the recent darker ATL re-imagining of kirk the better.
Anyways i think it would take a major POD to shift this. Discounting the comic book utopias of babylon 5 or escapist games like 40K
awn of Peace. Most serious utopian science fiction in OTL develops from the rebirth of the genre in russia after censorship laws were relaxed under gorbachev.