the problem with this is, I have no faith at all that the nitwits writing the show will actually get anything right,
They have the story end from Martin, so it is likely that they will get the ending right.
They will correctly hit the major plot points that Martin provided to them.
That's not the same thing as arriving at those plot points by way of a narrative that is satisfying and coherent or that competently resolves the arcs of the main characters.
Jon will end up on the throne. This has always been a story about him. The only way he wouldn't have ended up on the throne would have been if he had to die to kill the Night King (and you know save humanity). Even with as crappy as the last two seasons have been, I just can't believe they would get that wrong and frankly if Jon wasn't going to end up on the throne there would have been no reason for the super Mad Queen turn by Dany.
In what way?
If Game of Thrones has always been about anything, it's been about how the drive of certain people to gain and maintain power, be those people "good" or "bad," is inherently in conflict with the peace and happiness of people in general, and the impotence of "principled" or "moral" people to do much of anything to stop the chaos that results from the ruthless climbers' endless fighting for pole position.
In other words, "We can't have nice things."
Jon does have something that looks like a standard hero's journey, but so does Danaerys. To me it's fairly obvious that they are meant to be two sides of the same coin in this story, and that the ways in which they are similar and the ways in which they are completely different serve to advance that main theme.
It makes complete sense that Danaerys's drive for power would end up destroying the world instead of making it better, and that Jon's commitment to being "good" instead of seeking power for himself will prevent him from achieving happiness. Maybe he will kill Danaerys and prevent a reign of terror, but it wouldn't make sense for him to become the "good wise king who rules for fifty years" at the end of it.
I expect that the story will end on a hopeful note, but I don't expect it to seem like things will all be good and well for our main characters for the rest of time, or that the world is in a much better position than it was at the beginning. Except for the White Walkers having been defeated, I suppose.
I'm still not sure how the White Walkers or the Night King were supposed to fit into all of this. Maybe the books will make that clearer.