Of course, we as fans of the original series, hold a soft spot in our heart for the original actors. Hello, I doubt there would be anyone that could play Xander as well as Nicholas Brendon, or Giles as well as ASH- for me.
The cold, hard fact is that the series is old now; it began in 1997 and ended in 2003, and it's now 7 years after the series ended. If anyone ends up doing a remake, it's a whole new generation of people who will see it, and most of them probably won't know anyone in the original cast aside from SMG, and maybe Alyson Hannigan and David Boreanaz, and even if they do, they're probably not going to be bothered that it's new people in the roles the way we would be.
While I LOVE the actors who were on the show, I also love the characters. I don't want to see a "Buffy the Vampire Slayer" without Xander, Willow, or Giles. To me, they are an essential part of the Buffyverse and were much better characters than, say, Pike from the original movie. But for me, Buffy isn't Buffy if she's racing around LA or New York with some dude named Todd and her trusty sidekick Stake, the psychokenetic dog.