User blog comment:Winchester7314/Possible plot-hole, or maybe a deep hidden reason?/@comment-11787399-20131009180336/@comment-1509826-20131009181056

Indeed that seems to be the excepted answer, this show has long made it clear its possible to change something into something else with completely different strengths and weaknesses.

It is possible they could come back, as its implied significantly powerful beings could bring them back (why the would though is beyond me.)