Let's say that Nimoy decided to stay away from the franchise after the TWOK, but the producers decided to use the magic again properties to give us a Younger Spock body with the old Spock personality as a replacement. For Arguments sake, let's say they decided to use a young Robert Downey Jr. Would anyone have accepted this kind of move at the time?
Re: What would have been the reaction to a younger Spock from TSFS for When TSFS was released, fandom was in an uproar because they thought the Excelsior was going to replace the Enterprise. So, no I doubt a new Spock would have really gone over too well back then.
Re: What would have been the reaction to a younger Spock from TSFS for ISTR reading that even as late as 1991, one of the biggest sticking points with Harve Bennet's plan to do 'Starfleet Academy' was in part that while TNG had proved that a new crew could actually work, there was still a lot of uncertainty among the Paramount execs about the idea of recasting the characters Kirk, Spock and McCoy, and whether that would taint the TOS part of the franchise. With the benefit of hindsight we can see that it would have been workable, but I think it's really only been in the current climate of reboot movies that people have become much more accepting of the concept that iconic roles like that can be reinterpreted and recast. I certainly don't think the idea would have been welcomed a decade earlier. At least Phase II had the good sense to make Xon a new character. As far as the topic goes, I don't think it would have worked in context of TSFS either. The whole point of that movie was to give Leonard Nimoy an "in" to allow him back into the franchise. Who knows how it'd have turned out if he did walk away from Star Trek? Do we think the producers would have gone for a Spock-less Star Trek? Hard to say how that'd have turned out.
Re: What would have been the reaction to a younger Spock from TSFS for Could have been cool, if Shatner would agree to work with a lesser, young actor. When Wil Wheaton visited the ST V set The Shat shat his awesomeness on him. Roddenberry himself had to come to help. Then again, Wheaton in no RDJ. Yeah, Could have been cool. But we'd never know the great ST IV Spock. Alternate universes, indeed.
Re: What would have been the reaction to a younger Spock from TSFS for Wasn't that the whole point of the Saavik character? To be a younger, cheaper replacement for Nimoy.
Re: What would have been the reaction to a younger Spock from TSFS for If Leonard Nimoy hadn't been willing to re-assume the role of Spock, then TSFS would not have been made at all. There would have been further ST movies, no doubt, but they would have been completely different than the ones that were made.