To me, it felt like Dean taking Jack out was the perfect fit BECAUSE he's one who took the longest to come around and he's the one who was there before, living on borrowed time and deciding that fuck it, he was going to live his life out to the fullest while Sam was busy never giving up (and secretly Dean counted on him not to). In a continuance of Sam's new leadership role, Dean kept his hands free to do what he does, rally the troops and find a solution, while Dean was off giving Jack something he needed, kept an eye on him and found out that yeah, in addition to Cas and Sam who are quite sure in their roles, he is actually a dad to Jack too,

Well, I like this spin on it. I don't know that I can fully buy into it, because Dean has always been about giving himself the right to die, but doing everything he can to keep others alive, whether they want it or not. (I guess one exception is, like I mentioned in my review, when he decided Cas wasn't going to die a virgin and tried to get him laid rather than trying to save him.) But it still felt like Dean was replacing Sam, not joining him, as far as being Jack's father. Maybe another spin is that he was Fun Dad, and Sam was Serious Dad, the one who knows his birthday and insurance information.