It would be typical of the way Joss' diabolical mind works, but I don't think that he would actually do it. If the entire Buffyverse turned out to only be the fantasies of a demented mental patient named Buffy, there would never be any hope of continuing the series once more at a later date, and, while Joss might kill off every character in the Buffy series, he wouldn't destroy the entire universe, especially when "Angel" has his own series with another publisher, and there is still hope for more Slayer stories involving other girls.