Another way to save it that’s more plausible due to the guys aging some…. Would be that God could return from being off with Amara. God would return because there is a much greater threat from wherever God and Amara was that is coming for humanity.

So God brings Dean back to adult Sam in real time 5+ years later when Dean Jr is still a boy. Then Sam has his existential crisis because he fears he’s turning into his father.