I think the problem with Castiel - as much as I understand the corner of fandom that is very invested in him - is that he was supposed to be a character for a five episode arc that... spiralled out of contol. And nobody not even kripke had an actual conclusion for his story in mind, and they missed the turn, well a couple of them where it would have made sense which has led to a lot of... meandering. But thats a deliberation you gotta make with such an active and vocal fandom on your asses. We are a blessing and a curse.

As for the fucking up strategy, ive become very zen about that, it is what it is. Its my operating headcanon for game of thrones for example that they deliberately screwed the finale because they knew everyone and their grandma was going to watch regardless and that way, people have an actual incentive to want to read the books (that are never going to come out) in search for a better ending. But hee, what do I know.
It's not quite the same for supernatural of course, but well... the only thing that makes me really sad is how clearly jensen is telegraphing that he'll vanish behind the screen or into civilian life entirely. Not that I begrudge him the break, but still its gonna be weird not to see him regularly anymore.