It's funny, because this idea -- as bad an idea as it is -- kicks up a lot of dust that probably would have been kicked up eventually anyway.
1.) Buffy's an iconic character, and as such, eventually will pass out of Joss' hands. Probably even while he's still alive, although doubtfully while he's still interested. The character has embedded itself in the cultural matrix, and her story will get retold umpteen million different ways. That's how it goes for the icons.
2.) That being said, it doesn't seem the right time to do this, and certainly not while in a situation where they'd have to drop the other major characters in that story. At this point in time, "Buffy" without Giles or Willow is like "Superman" without Lois Lane or Jimmy Olson. It just feels wrong. (Even "Smallville" made use of Lex Luthor when it started up, along with "Superman" characters Lana Lang and Pete Ross.) Fans of the franchise are as devoted to the secondary characters as they are the titular slayer, and if the only thing that stands between them and a new take on the mythology is 20th Century Fox's scary lawyers, then ... well ... perhaps its best not to do it at all.
3.) Of course, the bare fact is that the "Buffy" mythos is being continued, in comic form, by Joss and writers under his direction. Now, I'm no purist about these things. As I said before, there'll be competing visions of any iconic character eventually -- take Batman, for example -- but at this point in time, anything not done by Joss will always be the knock-off, even if (on the off chance) it's brilliant. Batman had enough room, as a character, for an audience to choose between the comic, the animated and the film version as "their" Batman, but loyal fans of the character have few qualms about bouncing between the mediums, even if -- for example -- they'll always think of the comic version as the "real" one. "Buffy" fans aren't so starved at this moment that they'll automatically leap to a new product, just becuase it's there. (Take, for example, the many versions of the comic that failed before Joss got involved.)
4.) On the flipside, and still acknowledging Buffy as an iconic figure, one wonders if a movie would expand the mythology or deepen it. Surely, there were lots of bad "Dracula" movies, and yet Dracula's still around. (And even in Buffy!) There's really no way to know what the collective culture will do in that situation ... would it drive traffic to the comic or back to the DVDs? Would it kill interest in the character for years? Impossible to say. I don't know how fragile Buffy's mythology really is. I'd like to think it's fairly sturdy, but I'm not sure.
5.) Lastly, "Twilight" killed vampires for a while. When that series runs its cinematic course, no one will want anything to do with them for a while.