Dexter deserves a happy ending. The only happy ending is one where he stays true to himself - as insane as he's ever been - but with at least some people in his life seeing him for who he truly is and accepting him. At very minimum, I'd say that's Deb and Rita (and the kids, but kids accept what they're told is okay). That's enough for me.
Dexter will never be sane. It's implausible and it would violate the integrity of the character. The point of the story is that for him, insanity is