The studio definitely did fear that not enough Trek film customers would accept a Trek movie series without Kirk, but the main purpose of Kirk being in the
Generations wasn't really to try to turn TOS movie fans into TNG movie fans (although I'm sure they were hoping for that too). Kirk had to die for the TNG films to succeed, and I don't think that TUC would have worked if they killed off Kirk in that film. That was a retirement of the whole crew type of film that ended appropriately for that mood, a final adventure in which they start the long peace process between the UFP and the Klingons.
Kirk was there in
Generations to pass officially the torch from film captain to film captain. Kirk absolutely
had to die for that to fully take effect. If Kirk hadn't died then you would have had even more comparisons between the TOS and TNG movies with many fans constantly clamoring for Kirk's return. Kirk had to die to help Trek fans accept that he was gone for the sake on non-Kirk Trek. Kirk retiring and living happily ever after until the end of his days would just seem like a lame ending for such an epic hero. Kirk is dead. Whether you liked exactly
how they did or not, the torch had been passed from Kirk to Picard.
With that criteria, I think they did a decent job overall. They wrote a story that incorporated themes relavent to this passing of the torch. The title of
Generations not only refers to TNG as product identity. It also refers to the two different generations of starship captains whom the torch is being passed between. Since Kirk had to die, the film title also refers to a theme for Picard related to mortality: his new regret that he had never produced offspring to carry on the family history now that his only brother and nephew had died. Kirk served to help resolve Picard's sense of loss by reminding him that he was really making a difference as a starship captain, a lesson that Kirk learned in the previous Trek films after accepting promotion and feeling old and stangant without his command. This is all tied together by Picard's realization that his family history had lead him to a more important tradition, that of being an Enterprise captain. Thematically, this movie works.
Despite the fact that retirement was appropriate for the crew end of the previous film, it seems that for a character like Kirk, retirement would never do. It would make him feel old like being an admiral had. So it seems fitting that early in his retirement he heroicly sacrificed his life to save the new Enterprise. That seems a fitting death for an epic hero, at least how history remembers his death. And his first death had to be by saving the Enterprise-B,
next in the
generations of Enterprises after his two ships. And despite the fact that you may be disappointed by exactly how Kirk died his real final death, you have to acknowledge that at least they gave Kirk
two deaths in the film. Kirk is just too big for one death!
And there was no need for Kirk to return to the Nexus anyway, because he is still there and always will be...
Remember...
There was a part of Guinan that never left the Nexus, so we can assume it is probable that there is a part of Kirk still in there too. And since there is a timeless quality to reality in the Nexus, we can assume that the Nexus-Kirk is disconnected from the physical Kirk that left, so the Nexus-Kirk lives on in the Nexus beyond the death of the physical Kirk.
Besides the echo left behind being a way that Guinan could provide guidance to Picard from within the Nexus, it also served as a subtle suggestion of a "sci-fi heaven" of sorts for Kirk to live in for all eternity. So a part of Kirk is immortal...