That sentiment can only really apply to the original source material that the first four seasons stuck so closely to and the goodwill that's been carried over from that. Considering that everything after that has involved so many crowd-pleasing contrivances, it's really kind of a crap-shoot-- I don't fully expect a completely happy ending (If I were to bet, I would guess that ONE of Jon/Danaerys die out of all the main characters, and a bunch of less important side characters will die), but if it happened, it would not really surprise me or be out of D&D's M.O. to do that.
I sincerely doubt they have the balls to do a bleak, "everything's pretty much ****ed" ending (not that they need to).