As near as I can tell, Cole shows up in the episode after Cain, so you don’t have to worry about that. (Thanks to justanotheridijiton for the links.)
Cole is a very strong mirror for both Dean and John, and his story won’t be over until we find out what kind of monster his father was and how Cole reacts to running into it (or possibly being one). Dean made a point of never having seen anything like Cole’s dad, which means we’re going to see whatever kind of monster it was at some point later in the season, and I really doubt that someone who’s been on a revenge quest for as long as Cole was is going to be able to let it go as easily as it seemed like he did.
As pirrofarfalla says in this post, Cole’s role is to shed light on what makes a monster, and part of that is going to be what he does with all that rage that suddenly had no outlet. He was in shock at the end of “Girls Girls Girls,” completely thrown by Dean’s offer to let Cole shoot him, but that’s going to wear off, and then what?
He already knows there are demons and monsters, but he has very little experience with them. What’s going to happen when the monsters touch his own family? We know from earlier episodes that creatures that eat human organs can seem entirely human, even to themselves, into adulthood. It’s possible that Cole is whatever kind of monster his father was, and it’s one thing to understand that monsters are real, but an entirely different thing to learn that you are one yourself, or, worse, that your son is one. Even if you’ve already named yourself monster. If Cole can deal with that, and come out the other side without destroying himself and his family, maybe it’s possible for Dean to do the same. (Dean doesn’t think so, but the parallel is really for the audience.)
So Cole’s story is going to foreshadow Dean’s, or serve as a cautionary tale for him.
If you’ve read many of my posts, you probably already know that I completely disagree with you about the possibility of canon Destiel and that the 200th episode was mocking the fans, and believe the writers, especially Robbie, have a lot of respect for the fandom as a whole. I don’t know what else I could say that would convince you if those posts didn’t, and I’m sorry you feel so badly about it.