They're raping my childhood!
Rabble rabble rabble rabble rabble rabble.
Don't think that comparison is really in the same ballpark. The Star Wars prequels:
1) were not a "re-boot", telling a different version of Luke Skywalker's origin as a Jedi. They expanded the existing story's universe, not "re-imagined" it*
2)
were still in the hands of the original trilogy's creator, for good or ill
3) did have the benefit of enough time passing that there was a nostalgic childhood rememberence, amping excitement about the new movies
*midichlorians notwithstanding
Sure you can boil it down to "the sun is going to rise; fans are going to grouse", but I think the third point the most critical - it's going to be recent fans grousing, whose memory of the show is still fresh in their minds.
Yes, some may see the movie to compare/contrast/complain, but I have a feeling a large portion of existing fans won't be interested, or at least not enough to see it in theaters, or buy it on DVD. It'll probably be a cable/rental/borrow situation for the curious among them.
Of course using "brand names" is usually less risky, but again, in this particular case, I think it's the use of that brand plus the fact it's also a re-boot that makes it problematic. It might actually be somewhat more risky than going with an original character and perhaps skewing the marketing with hype like "a worthy successor to Buffy" and what-not.
Also, the comparison of re-booting Buffy being like Nolan doing Batman? Nolan's "Batman Begins" had the benefit of not only a decent number of years since the last Batman movie, but also the fact that the last couple of prior Batman outings in the theaters sucked and the character needed a fresh take. Again, neither is really applicable to Buffy.