round and round the winchesters go

Essentially this.

End!Cas is the product of a separate universe/timeline controlled, if not created, by Zachariah (and look at what he did to Dean and Sam in 4x17--do you really think we should trust what he makes of the characters in his worlds?). He's a "careful what you wish for", not a "this is/could be what Cas is really going to be like" meant to point out how 5x03's type of friendship, how Castiel choosing Dean in general over the angels, will drag Castiel down to Dean's level of everything he hates in himself: and I mean, exactly Dean's level. #Drugs, #alcohol, copious amounts of detached sex, #suicidal nihilism, and #codependent loyalty to an aggressive, stoic, seemingly unfeeling leader, are all so Dean it hurts, and that's where "Don't ever change" comes from. It's Dean releasing Castiel of his attempts to make Castiel exactly like him.

Additionally, on the other hand, it also, yet again, means nothing about our Cas, possible or not, but drive. I talked about this here: Castiel is very deprived of affection, touch, and appreciation in general, and it's not super impossible that, being sex-indifferent but not -repulsed, he'd go for what Dean taught him in order to get some of that ("hedonism" of just "I need to feel good somehow when everything sucks and oh look there's warmth and companionship baiting this hook" just like he described w/ April) in a apocalyptic, doomed context (think Dean/Anna in 4x10); no attraction necessary, only drive, which I already acknowledged he has.