Except that the amount of time isn't irrelevant.
You can't reboot something too quickly, else it can hurt the market. You need to wait at least four or five years to reboot a franchise, with absolutely no coverage, else you risk confusion in the audience and glutting up the marketplace, as well as damaging the brand. This is true for THE INCREDIBLE HULK, STAR TREK, CASINO ROYALE, and even upcoming reboots like the new SPIDER-MAN and SUPERMAN movies. This is because there are two reasons to reboot a franchise; one, to refresh the franchise after a failure (such as BATMAN BEGINS or any of the ones I just mentioned), or to relaunch a franchise when it's out of date (such as STAR TREK, which is both types). Spin-offs and continuations are different and can be immediate, and the cast only changes if too much time passed between the original and the continuation (such as SUPERMAN RETURNS) or it's built into the mythos that the characters change (LAW & ORDER, DOCTOR WHO). (There's always an exception though; SMALLVILLE and SUPERMAN RETURNS for example, or THE BATMAN being immediately followed by BATMAN: THE BRAVE AND THE BOLD.)
Now, none of this has anything to do with the quality of the reboot (which is a different matter) but everything to do with the reception of the news of the reboot. Time has to pass when rebooting a failure in order for the bad taste of the failure to dissipate, otherwise people won't fork out $10 to see a crappy movie that they just saw. But time also needs to pass in order to relaunch a franchise when it's out of date, otherwise, if it's contemporary and still effective, it would still be ongoing.
In terms of the BUFFY reboot, while time has passed, it's being rebooted for the second reason (which I understand) - teenage romance with vampires is cool at the moment, and BUFFY is an iconic franchise of that genre (it pretty much popularized it). However, for a franchise to be out of date, it needs to be from an earlier generation (which BUFFY isn't) and/or the original cast and crew is simply incapable of returning to the roles (which they aren't). David Boreanaz, Sarah Michelle Gellar, Eliza Dushku, Alyson Hannigan, Seth Green, and Joss Whedon are all in work and doing very well and are more or less at the same popularity they always were. So you can have new BUFFY stuff with the old cast and crew and that's why not using them just seems 'wrong'. While it's certainly true that they couldn't pass for teenagers so much, a big BUFFY movie with the original cast and crew is not only doable, it's desirable by the fans since the show never fell from grace.
As I said, this doesn't mean the reboot will suck, but it does explain why the target audience is reticent about the reboot because it's simply too soon. Time is specifically why people are annoyed (and it's specifically why it's being rebooted now). If this reboot were announced in ten years, people would complain, but the quality of the complaints would change from offense to skepticism because people would just get that the actors would be too old (particularly Boreanaz and Marsters) to continue their characters, just as people were when STAR TREK was announced because everyone had enough of the extended universe (thanks to ENTERPRISE and VOYAGER) and the key actors not pushing 80 are dead. People were worried that the reboot might suck, but no one went, "Why can't they just get back the original cast?!"
Time is a key component in not only what type of reboot a franchise will have, but how people will react to the news of a reboot.