This man that you've clung to your whole life because he's the only source of safety in a terrifying world you've ever had and the only person who's ever understood your grief and because he's your DAD...
He never ever apologizes you didn't even think he even saw, but he did see, and he apologizes.
You buried every resentment you ever had for him your whole life because you felt you had to to hold your family together even when you were just a child. Your brother was screaming and screaming and you were silent. You didn't complain. Not because you had no complaints but because you buried them. You comforted John instead when he should have been comforting you. And that was its own source of resentment too. But you never ever voiced that—you never let yourself think about any of it whenever those thoughts started to come because they'd destroy you.
But then your Dad voices them TO you.
HE says he put too much on your shoulders. HE says he made you grow up too fast. HE says you didn't complain not once, implying that HE knows you could have and you were silent for HIS BENEFIT. HE says he's sorry that he was just a shell—that you had to comfort him when you were just a child and HE should have been comforting YOU. And these are all things Dean has never allowed himself to dwell on, but John is voicing them. He is speaking them into existence. And by speaking them he makes them extremely real to both of them, and he has the power in that moment to begin healing Dean or harm him further because of the vulnerability he's created by speaking those words. And then he condemns Dean with them.
The apology isn't the precursor to a change. It isn't John promising things will be better from now on. It's John's acknowledgement, before destroying Dean, that he knows it isn't fair. It's him saying that he knows he hurt Dean over and over his whole life even though neither of them ever acknowledged it, and he's about to hurt him again—maybe worse than he's ever hurt him before.
His last act will be to cement Dean's parentification forever—to make Dean responsible for the course of Sam's entire life going forward and whatever consequences that life brings to the world. It's all on Dean's shoulders now, and John doesn't even tell Dean why. Just like so many times before, he leaves his sons with more questions than answers—leaves them fighting in the dark—this time in the cruelest way he ever has. And he knows it's cruel. He specifically speaks to exactly how cruel it is in detail before whispering those words in Dean's ear. So Dean is left with the memory of a man who didn't harm him in ignorance but knowingly harmed him over and over and only apologized as a means of saying how sorry he was about to do it again on the way out.
It also shatters Dean's family. It cements the knowledge that none of them will ever be whole, John will never be there to fix what he broke. He'll shatter Dean on the way out instead.
