Some people will need a lot of time in the gym to do it. BUT.. as incredibly boring as the gym is (I assume, I've never ventured there) if you are being paid to add years to your life by exercising and you get to be in a major motion picture after conventions being your bread and butter for years wouldn't you do it? Well, I would.
Because in the future everyone looks fabulous "25 years later".
How would they go back and fill in the gaps with the same actors?
I would like to see them in the future post Endgame, some kind of AU is fine with me. I don't really care about filling in the storyline as I'm satisfied with the novels but getting the crew back together and forcing them into a storyline (even though they would have scattered in their careers) they way TOS films did, that would be a wonderful thing.