On a metalevel, TFE did exactly what it was supposed to; it brought the series to a resounding end. (Perhaps I'm just seeing the writing leaking around the edges.) So it certainly succeeded there.

Which is to say, I agree, it's a good theory. TFE certainly did change the nature of The Slayer(s), in a semi-permanent -- or, without Fray, permanent (I haven't read Fray, which made the appearance of Cool Weapon a surprise, and which led me to believe there would be no more Slayers, ever) -- fashion. The idea of Evil investing for the future is a strong one, and appears in many other forms of literature....definite precedent.