I think not keeping Decker, Ilia, Saavik, and David Marcus around longer was a missed opportunity.
This! Although it was hard enough to get enough screen time for the classic characters from the series, adding in more regulars would have really condemned them to the back.
A shame, really, since they were all created as very dimensional, intriguing characters.
As an aside, I wonder if George Takei's motivation for Sulu's promotion wasn't motivated in part by the chance to have screen time without Shatner. Just wondering.