When the first movie was planned, a major business decision was made to treat Sulu, Chekov, Uhura, and Chapel as important characters who were going to fly with the Enterprise wherever it went. That was good for those actors but I think it was bad for STAR TREK.
I can totally see treating Scotty that way; he had by far the most important job of the group, and it was a job that a very senior man could plausibly stay in.
But the other supporting characters were people who simply would not still be with Kirk after ten years and more. Assuming they all stayed in Star Fleet and were all competent, they'd have to climb the ladder and become commanders at least. Which they did.
And if you keep them on the same ship with Kirk and Spock, you now have a crowd of "commanders" with no subordinates on the bridge. They had to be still pushing simple buttons on Kirk's orders like a kid out of the academy despite their supposed high rank. It strained credibility (as did their rusty acting skills).
Just as those supporting roles were cast with young people in 1966, so the movies should have brought in young people with new character names in 1979. From the TV series, they should have kept just Kirk, Spock, McCoy, and Scotty-- both for plausibility and because they were the best actors by a mile.