My peer group of rabid Trekkies was early 20s at the time. Most of us figured that TOS was dead after TFF. I mean, it was SO BAD that it was the first Trek film any of us didn't see even a second time. TNG seemed to be going so well that we all just assumed that "the winners" would get the next film. It was pretty surprising when it was announced that they were rushing a film out for the 25th with the TOS cast. But, when Nick Meyer was announced as Director, we all got immediately excited for it. We remembered what magic he worked with TWOK so anticipation got pretty high and he didn't disappoint. In the end, it was really good that the film was made so the TOS cast could go out on top. We all knew TNG would be the next film, it just seemed inevitable. We were just sad that TNG had to end on TV to make it happen. It was SO short-sighted of Paramount to think that a movie couldn't be good promotion for a current TV show- something Fox proved later on with The X-Files.
That montage trailer may be one of the best Trek ads of all time. Love that final comparison of Shat and Nimoy from WNMHGB and TFF. Yes, Christopher Plummer did the voice-over. I first saw it late in the summer that year when TMP was reissued on VHS in letterbox. That trailer was on the tape.