So, if Jack Black....yes that Jack Black--as is--slipped on the pointed ears, delivered his lines in a monotone voice and sported the haircut, he would be Spock, just as much as Nimoy?
Sadly, yes. Yes he would.
The problem is that Star Trek is a franchise. Star Trek is not defined by what it says, how it looks or who the actors are. Star trek is whatever the owner of the franchise says it is. If they did a Jack Black as Spock comedy movie, that would be Star Trek. If the did a Quinten Tarantino bloodfest, that would be Star Trek.
Star Trek exists for one reason, to make money for the trademark holder. That's it. It's not there to push the boundaries of science fiction or to tell little morality plays in a space setting. It's to make as much money for Roddenberry or Desilu or Paramount or CBS as possible. If what they do is popular, that makes them more money. If it's not, they make less money and they try something else, either a new setting line TNG or DS9 or a reboot like JJ's version.
So the answer is yes, Jack Black could be Spock. I don't think he'd make a good one but my opinion doesn't matter. If enough people like him as Spock then he's Spock until they get tired of him and Star Trek moves on to something else.
Such is the entertainment industry.