Yeah, I wondered the exact same thing. And I can't help but think he intended this very ending anyway, even if it had been series end and not season end. In fact, I've seen some say that it was likely Dean would have been Michael after all, and both he and Sam would have ended up falling into the gate. And to be honest, that seems very plausible. I could see Kripke ending that way. And that would have hurt a hell of a lot, too.

Agreed to your second comment, too. Man, it's just been on my mind all morning, and I still can't untangle my feelings about it.