I see your point but in a way there was a turning point in the story of the TOS characters in the movies but it was not in STVI it was much earlier and I hate to say it but in some ways this is Nimoy's fault.
Hear me out, I love the guy and I am glad they brought him back in STIII. But I get the impression that STII was introducing the idea that the old crew was making way for a younger crew or as Kirk put it "galavanting about the galaxy is a game for the young." Now the movie proved that he was not that old, but in later movies they might have passed the torch slowly. Spock had already did that with Saavik essentially. We would have likely seen that with other characters in future movies.
NOW Spock came back and in the process the Enterprise is destroyed and in a way the movies just seem to try to give the crew the proper send off. STIV was a possible ending but the pushed their luck with STV and failed so they tried for one big send off with STVI. It does a great job But as mentioned before there about ending the story not really about growing the characters. There is a strong need to put a bow on the saga. It works in the immediate but as one reflects on it the sadness of what might have been sets in.