I'm from the same neck of the wood (as you probably know ;)) and yes, it does seem like getting rid of Cas - either by killing him off or simply sending him back to Heaven never to be seen again - would lose them a lot of viewers. But, to be honest, I'm not sure how people in the majority of the fandom think about it, I don't exactly get there much.

Killing Bobby doesn't sound like a much better idea if they don't want the wrath of fandom. Then again, people wouldn't probably stop watching if Bobby weren't around any more.

I've never watched Smallville but I've heard bad things about the road it went down, and I certainly don't want that for my show.
Can Ben Edlund please take over already?