> Riker does spend a long time on the Enterprise, even after being offered command a few times. He does eventually move on.

Also he is not free of ambition, he wants Picard's job. He isn't willing to do anything negative to get it, but part of that is that Picard constantly proves himself the better captain. Picard isn't captain because he joined Starfleet first or because he has better connections, he is genuinely better at the job and non-toxic ambitious people can respect that. Riker still has more to learn from Picard that will make him a better captain later.

So I think a lot of the people staying in their positions make sense, the primary characters are as high as they can go (for their specialty) without transitioning into a role that stays on a planet.