User blog comment:Winchester7314/Possible plot-hole, or maybe a deep hidden reason?/@comment-11787399-20131009180336

My guess is that Death was saying souls couldn't be "broken" in their current state (pre-demon). Since Demons are transformed, not broken, from souls, I guess they lose that whole soul immortality aspect of it, & can be killed in their current demon state.

Oh yeah, Meg's not a big baddie either, she's the only one I could see the writers bringing back though. The other 4 you listed served their purposes during the Kripke Era, only plausible way for them to show up again is through an illusion (like Azazel in "Exile On Main St"), or though some new flashblack that explains some future plotpoint or connects all 9-10 season together.