Okay, like many others have said, this needs to be canon! I can totally see Kripke and co. going there too! I'm guessing maybe all Michael!Dean needs is the spear (that holds his grace somehow?) and he can use that to defeat Lucifer and never has to revert back to being Michael. Yay! I kind of want to write a fic based on this idea now! Not sure how you'd feel about that, but the point is I'm inspired!