I’ll disagree.And for the record (sorry for the side rant), I think the end of GoT is Martin's issue, not D and D. The reason he hasn't finished the books is because he can't figure out a way to get the characters to do what he wants for the story he wants to tell. D&D just said forget it, we'll force it so we can finish. Unless Martin decides to allow his ending to change, he'll never finish.
Deneareyes ending failed because it was too sudden and came out the absolute dumbest moment possible. Take some time to stretch that out, and put some real pressure that causes her to have to make the “key evil decision”, and I could absolutely buy it.
Jaime - also can work in the context of a “drug addiction”. Cersei is his drug, and drugs bring down great men. His ending is an utter tragedy but completely workable, just don’t have him bed and leave brienne right before to make him look like an utter douche.
Bran- just have bran do anything (and I mean anything) to help beat the apocalypse, and suddenly you have some real cred for people to consider him king. Or go dark side and have manipulate to become king behind the scenes…either works.
The ending can mostly work if you actually spend some effort and time to work the angles