Janeway didn't achieve jackshit and became Admiral. Cadet Kirk essentially only stopped a terrorist and got promoted to Captain and got command of the flagship. By the these movie's logic, Picard, Kirk, Sisko, Riker & Co would have been elected to Federation Presidents or Masters of the Universe long ago.
I think Janeway was promoted to keep her close to home. She returned home in 7yrs and crippled the Borg, which would have made her famous. But after some of the crap she pulled in the Delta Quadrant and what Future Janeway did, maybe they wanted to keep her stuck at a desk.
As for STXI, that was really stupid. They should have flashed fwd 6yrs and made him Captain then. They are really setting the young up for a major disappointment. The real world does not work like this. You cannot go from Cadet to Captain. Of course they followed the Hollywood mentality. You put out a good hyped song, movie or TV show, you become a star. Hollywood offers you roles you're probably not ready for, simply because you're the latest thing.
As for Kirk, I think the writing was on the wall. The Constitution Class was being decommissioned. I'm pretty sure Navy crews are trained for specific vessels. They can be retrained for other assignments, but that is probably an expensive process. His command crew was moving on in their careers. So he probably decided it was time to retire.