My gut reaction is that this would always have been a very fine needle to thread.
I only ever watched Buffy casually, but to my mind the show's theme was very much about adolescence; that the struggles Buffy and her friends faced were essentially metaphors for the difficulties involved with being a teenager. With Buffy herself (and, if they brought them back, her friends) now being middle-aged, that wasn't going to work anymore. The show would either have had to change its theme to tell the story of going through a different point of your life, or it would have needed to transition to a newer, younger cast. The former risks alienating its existing fans because of the change in the show's theme, and the latter risks alienating them because it discards/minimizes the cast members they've ggrown to know and love. Either way, it was always going to be an uphill struggle.
And that's not even taking into account issues with the canon, which I'm sure the fans would also have at least some investment in. Like, I suspect that no one thinks the novels would need to be taken into account, but what about the Buffy comics? You know, the ones that labeled themselves Seasons 8 through 12, and were presented as being the official continuations of the story? Which included things like magic being banished from the universe and then restored, changing how vampires work? Or which had Spike gaining possession of a spacecraft?
Those are just a few of the issues involved, and while I don't think that any of these are necessarily fatal unto themselves, I suspect that altogether they made the prospect of a new Buffy show one which was nice in theory, but far less than ideal in reality.