It was time when the idea of a Star Trek without the familiar characters or those characters played by different actors was a notion too bizarre to contemplete. There had been some talk of recasting the characters for a feature prior to TMP, but it obviously lost traction as TPTB likely realized what a foolhardy move that would have been particularly given the availability and health of the original cast.
Today the films are a done deal, but back then fan speculation exploded over what could happen in each successive film. WTF? They killed Spock??? They destroyed the Enterprise??? How can they possibly go forward from that? How can they possibly make it work now??
I remember those days too! Before ST IV came out, I thought they should make Kirk and crew an elite force of some kind - with a small ship and tasked with important, sensitive missions. After they did something amazing to get pardoned for their actions in ST III, of course.
I also remember how unexpected The Final Reflection by John Ford was. A novel, that, except for the brief framing sequences, featured none of the TV cast, with only cameos by Sarek, Spock (as a ten year old) and an infant McCoy.
There was a freshness to Trek back then that sometimes seems diluted by the many iterations we've had since.