And of course we must not forget the single most important reason Buffy had to "free" all the Potentials and make them Slayers; the franchise needed to stay open. The one girl in all the world became hot chicks with superpowers. That truly is the driving force, IMO, philosophy and feminism be damned. It was a smart idea to keep open the possibility of future series, comics set in the present, etc., w/o the driving need to deal with the "one girl" concept and Buffy, it's embodiment.