Here is my biggest problem with the ending and it goes right back to their effing mega nonsense of 'God wrote everything'. Except for the very few times when the biggest of situations would arise, the guys never had free will. Free will only seemed to prevail in the most dire circumstances (Dean not killing Sam in S4, Sam not killing Dean when possessed by Lucifer, the end of 15x17).
We know that Chuck wrote everything down to their dialogue and even sometimes thoughts. Remember when they met him in S4 and were at the laundromat, Dean had written pages from Chuck which he read out loud that detailed their conversation in the laundromat right down to even their thoughts. Everything about these guys was scripted with the exception of the previously mentioned extreme circumstances.
So to finally be free from the scripted pages and Dean dies a few weeks later? That is not just the most depressing ending but it's downright diabolical.
Also the only reason why the supernatural existed on the earthly plane was because it was part of God's story. So what, NUgod decided to leave the evil from heaven, hell and purgatory able to run around the earthly plane to continue to kill people? Or is it just purgatory? There shouldn't be any supernatural interference on Earth anymore. There shouldn't be vampires at the end of this story, period. And that's how the show should have ended. These 3 planes have their access to Earth completely cut off and what's left of them on Earth gets send to their respective plane.
Then the guys should have to struggle with real life while also being completely free for literally the first time in their lives. After all, the writers decided that once free of God's writing, the guys went after normal things instead of continuing to hunt (Dean's job application certainly points to that). Which means that God was the one preventing them from ever really having that in the first place. The occasional interludes with normalcy that didn't last seemed to have been God's manipulation in a 'see Dean/Sam, normal always ends in disaster for us so we keep hunting' kind of way. It was not their decision to keep hunting year after year. And if during that life of normalcy one of them slipped on a banana and broke his neck or fell down an elevator shaft, so be it. But literally being free to speak, think and decide for themselves for the first time in their lives, to kill one of them that soon after the achieved freedom...I just can't.