Temis the Friendly Ghost wrote:
All the characters should come and go, that's what makes it realistic for the military.
That is what is realistic for today's military. And not knowing anything about non-American military habits, possibly even just the American military.
A hundred years ago someone joined a regiment and stayed in that regiment. You were promoted, if an enlisted man, when your captain felt like it and demoted when he found someone better. You were promoted, if an officer, when the guy above died or retired or when the guy above him died or retired.
The modern approach to constant rotation developed after WWII when the military consciously attempted to make sure troops were used to working with new people to eliminate the "Doomed Replacement" stigma, because draftees were often around for a little while, while the unit never moved, and because of the rather interesting "up or out" policy where any officer who isn't good enough for promotion is kicked out even if he is great at his current job.
Starfleet is NOT the U.S. navy in space. I don't know why people think it needs to operate like it. If nothing else, the fact that it keeps it ships in space for years on end means that it makes sense to keep crews together for that long. I remember reading that when TNG started there was an idea that the Enterprise was on a 25 year mission--if that was the case, why would they launch a ship that would need to transfer all of its crew away every couple of years.
Ignoring the idea that realism means copying the U.S. Navy of 2012 for any spaceship show, Starfleet has consistently shown that it has no problem with crews staying together for years on end and with people refusing promotion and staying in one job indefinitely. Riker may have jeopardized is career by staying under Picard, but the only time that ever threatened him was when Shelby implied that he should
get out of the way, not that he would ever be forced out.
Kill off characters when the story demands it. Don't lock yourself into a set cast of seven for seven years. I agree with all of that. But don't change the cast to seem more "realistic."