Here’s my take on the ending:
Since the start, Sam was supposed to be the Main Character™ , but since Jensen created Dean in such a beloved way, and later Misha did the same with Cas, the writers were forced to give them more attention. Jared was always displayed first in the credits, and Misha wasn’t considered a main character until like season 9 or something, so its clear who the showrunners prioritized. When Cas was killed off in season 7 the show’s ratings dropped by an outstanding number, so the writers scrambled to write him back into the script. Misha/Cas AND Jensen/Dean always had the hearts of fans, and the writers and creators didn’t like that because Sam was suppose to be The Guy.
There’s no way Misha “do what’s right even if it’s against the law, death to normalcy” Collins actually liked the way this ended. The fact that the cast never really gave Misha a proper send off, at least not publicly, like they did with every other character on their last day, is suspicious. Maybe there was supposed to be another Cas scene but Misha got into a fight with the writers. Who knows. I wouldn’t be surprised. That could explain why the episode is so significantly shorter than usual. And Jensen also said there was a cut scene from 15x18 that he didn’t know was cut until the episode aired. So it’s totally reasonable to think that a Cas scene was cut from the finale, possibly due to a dispute. Misha is clearly the most bold and imo its because he knows he doesn’t get paid enough to let the writers shut him up and knows that he can’t get fired because the show will go under if he’s killed off (ex: season 7 crisis) so he’s ranted about the show’s faults many times in conventions and thank god for that.
I was sort of shocked when I first found out that Sam was written to be the head honcho but it makes sense if you watch the early seasons because Sam is the one with the actual explained backstory, the “rebellious son”, the one who gets psychic powers, became soulless first, etc. Later, since people leaned towards Dean more, he got more elaborate plot lines, like the Mark Of Cain/Demon!Dean, the purgatory thing, and the whole Amara thing. And since people ADORED Misha/Castiel, they gave him soooo many story lines that weren’t the main ones but still were fantastic. They literally made Castiel God, he fell from Heaven and became human, and Cas was ultimately involved in a lot of turning points in the show. In my opinion Castiel is the most complex main character, and I’m not surprised that Buckleming and Dabb couldn’t handle him. Dean was supposed to be basic, but Jensen basically said “fuck you” to the scripts and used his acting skills to add depth to the character, and for that I literally cannot be more grateful for Jensen Ackles. Misha did the same thing but in a different way.
Andrew Dabb has repeatedly expressed his dislike for both Jensen and Misha. Jensen and Misha took the characters and made them their own, into something the fans LOVE, and that pissed off the writers because Sam was supposed to be the most beloved one and the complex one, Castiel was supposed to be a temporary character and Dean was supposed to be the dumb goofy one, and honestly I think it’s all just a control thing. The writers and showrunners had no control over Misha and Jensen. Which honestly? Is the plot line for Chuck. To paraphrase, Chuck basically said, “in all the other worlds, you did what you were supposed to do. In every other universe you played by my rules. But not you.” That was Chuck’s whole thing. ESPECIALLY about Castiel. Chuck said, about Castiel, “You know what every other version of you did after ‘gripping him tight and raising him from perdition?’ They did what they were told. But not you. Not the ‘one off the line with a crack in his chassis.’ All of you. You know, I tried and I tried and I tried, but you're all just too stupid, too stubborn. Too broken. You know what? I'm over it. I'm over you.” This is literally the writers reflecting their true selves. Chuck is literally them in every way. They made Chuck a writer in the beginning, and later revealed that he’s God, so......honestly, this last season was literally just the writers spewing their feelings onto the scripts.
(I’m not saying Jared didn’t put any effort into Sam, I’m just saying that the writers always viewed Sam as the main man so they probably liked him the most and seeing the others get more attention from fans was probably annoying. Sam is also a complex character, but he was always written to be that way. Dean was not supposed to be so complex, and Castiel CERTAINLY was not supposed to matter as much as he does. I love Sam, but this is what I think the writers/showrunners were thinking when they decided to screw over Jensen and Misha/Dean and Cas.)
(Also I’m not an expert, this is just speculation, but I think this is a pretty solid theory)
TL;DR: The writers (specifically, Dabb) didn’t like Jensen and Misha because they couldn’t be controlled and rose above Sam in many cases, and they used Chuck as a way to reflect that, and used the finale to basically send a big “fuck you” to Jensen and Misha.