I agree with everything you say! I guess Dean's looking at a long life of becoming a demon, getting cured by Sam, becoming a demon, getting cured by Sam... until eventually Sam dies, and then Dean just becomes a demon.

Though, I guess Dean's real fear is what Cain said about killing Cas and Sam eventually.

And yes, as clever as Sammy is, I think Rowena is more devious and will eventually get her way.