Alright, so I have too many thoughts to express right now, but I'll just start with some basics and a little bit of a rant. Actually no it's not a rant because it's kind of optimistic.
First of all, what is up with people saying that this is three years of character development down the drain in relation to Castiel. This is not an end in any way, guys, and I hate to break it to you, but by assuming that this is the end of his character is a hell of a leap. Remember dark!Willow, anyone? Yeah, just saying. This is setting up for some fantastic recovery and character development in my opinion.
Starting with the Misha rumor (AND WHO KNOWS IF IT'S TRUE YOU GUYS WE HARDLY KNOW ANYTHING STOP FREAKING OUT SO MUCH). The fact that Misha isn't going to be a "series regular" next season, doesn't mean he's gone.
Let's just think about what's happening in comparison to previous events on the show. When Sam was drinking demon blood, how was he cured? He was locked in a room by his brother, forced to purge the dark, the evil, from his system.
No matter what Castiel has said about Dean being like family to him, his real brothers are in Heaven. His family, both the ones that stood beside him and the ones who didn't. Castiel needs an intervention, needs someone to do for him what Dean and Bobby did for Sam, and the one to help him, can't be Dean. With the power Castiel now has, Dean won't be able to help him. He won't be able to talk him down from this surge, this godly platform he's standing on. Dean had his chance and he didn't take it.
We only got a glimpse of what the tearing down of the wall did to Sam, but I can guarantee you that Sam will need Dean more than ever next season. I can also say with some certainty that Castiel won't be staying on Earth for longer than he has to. We know how God was portrayed in previous seasons; absent entirely, not bothering with the problems of the human world. Whatever Castiel needs to do, whatever he needs to experience, whatever can get through to him, to who he used to be, is not on Earth.
I honestly think, no matter what Gamble says about characters other than Sam and Dean being cycled out of the show, that this won't be left unresolved. Honestly, I felt like the end of this episode was the start of something, not the end. Castiel was not completely cold in his decisions. He had doubts, misgivings, and maybe he didn't realize them in time (no thanks to Dean), but this was not his plan. He's had a taste of power, his own demon blood, if you will, but that doesn't mean he can't recover.
It doesn't mean he won't come back.