I can't remember who wanted to do a Starfleet Academy film about Kirk and Spock's Academy days, but I believe it was Bennett. To me, that would have been a fun way to go, allowing Kirk and Spock to reminisce and show us a new adventure, while allowing the aging cast a little less action-yes, you would have to argue with Shatner on that but there are ways.
I like dealing with the loose threads from TVH, which largely go unremarked and unresolved due to TFF. The idea of Kirk's "mysterious death" would be an interesting one, but it focuses a lot on Spock as detective, and deals with a lot of information already known-in other words, it really isn't "new," at least in the sense of a new discovery. Say what you want about TFF (and there is plenty to say) but the idea of Spock's half-brother? Kind of crazy, wouldn't you say?
Also, let us all keep in mind that the studio wanted a comedy for the next Trek film because TVH did so well at the box office. Shatner wanted a introspective, philosophical work about finding God, which is tough to do a comedy about. Not impossible, mind you, but really not Trek's thing. So, the film was an attempt to bridge both concepts, and really lets us down by not really being either or face palmingly awful in execution (Scotty's head knock, anyone?).
Personally, I would have preferred and Academy-era film, or a film working with material from TVH. If they had done so, we may not have had to go with TUC, since we could have an attempt by the Klingons to bring Kirk to their version of justice (kidnapping, or something like that) and him to deal with the consequences of his actions.
Despite the aging of the cast, I see no reason to stop making films, but to have a willingness by all parties to acknowledge that aging, and to try something different.