I personally think Trevor and Franklin will survive, not Michael however. Even though he has more to lose, if you play the cult parody of scientology missions you would realize that you are given the choice of giving the money to the chopper, or destroying it and taking the money for yourself.
Here is why I think Michael might be dead by VI. In the real world, Scientology has been responsible for an uncountable amount of deaths relating to money for the church.
If they choose that option for the next game. Heck, even if they didn't, Michael was still part of a dangerous cult, and once they found out how much money Michael had, they would find someway to kill him so they could take it.
Scared for their lives, Amanda, Tracy, and/or Jimmy would go into witness protection again which you may see them in the next game doing.
Trevor however, you know how GTA leaves clues about the fates of their protagonists, how Michael labeled Trevor a hipster. I have a feeling Trevor may eventually meet the right girl and clean up his act and stop using drugs and get a job as a Barista at a coffee shop in the next game.
Your character may be on a mission where he meets this guy at a coffee shop and serving lattes behind the counter is Trevor.
Call me crazy for saying that but I don't think Trevor would continue with his crazy ways especially if he finds a girl who is crazy but in a good way.