It would have been more realistic for Spock not to coincidentally return to his actual age. However no-one has every played Spock better than Nimoy and noone ever will. Even cute little boy Spock. Aside from that imagine Shatner having to compete against a younger man.
I've always sort of assumed that Spock was younger than he looked (proportional to Vulcan aging) in TMP and TWOK and that the Genesis Effect actually made him age past where he'd been before. After all, he was only mid-50s as of TWOK, and we know from Sarek, Tuvok, and T'Pol that a Vulcan should still look like a human in their prime well into their 100s. Let's see, Spock was 157 when he went back into the Kelvin Timeline and survived another 5 years, so he died of natural causes at 162, which is actually pretty young for a Vulcan. Sarek made it to 203, which was a typical Vulcan lifespan according to "Broken Bow" (although The Making of Star Trek set Vulcan life expectancy at 250). So that's consistent with my notion. Although Spock being half-human could also explain his faster aging and shorter lifespan.
I disagree, I think it would have made perfect sense. The rest of the crew hadn't been regenerated from dead tissue by a terraformer-gone-crazy It was less 'realistic' to bring back Mr Nimoy in that scenario. And that also precluded a lot of interesting possibilities ( Kirk vs younger Spock as suggested, Sarek and Amanda struggling to accept a 'new' son, relationship with Saavik ) Instead we were hit with the Universal Reset Trick. What a cop-out.
But there was no reason to bring in a new actor as Spock. What the studio and audiences wanted back then wasn't just Spock, it was Nimoy as Spock. Anything else would've been financial suicide.
Well they have accepted Zachary Quinto as Spock, although he does look a helluva lot like our Len but will they accept Ethan J.Peck in DSC? Time will tell... JB
I was talking about in the context of the early 1980s. It's a very different thing to cast someone else in the role when you're telling the story of a young Spock and Nimoy is in his late 70s or deceased. The question was about recasting Spock for Star Trek III at a time when Nimoy was active and available, and no, I don't think it would have worked.