Well, the point was for 7 books and twn years of your life you read a series where the author took traditional fantasy tropes and turned them on their head. The ending is just another aspect of what Bakker did with this series.
But, you know, there is still a 3rd series to be written to finish off TSA. And, i will tell you if it is juat about the destruction of a civilization with no hope...well im not gonna take the time to read it, myself. But, i dont think that is gonna happen. Either Kellhus Tricked the Trickster and will aid humanity through the Outside or it will be a tale of human perseverance. And, i think the latter is much more probable, imo. TSA is a warning to us about what might happen if we let Technology rule us, Nonmen and Inchoroi are both analogs for posthumanism. I think what he did with the 1st 2 series was well done and flipped fantasy on its head. When have you read anything where the bad guys actually win? The hero wasnt a hero after all? Maybe it couldve been done better, i dunno.
Do remember, Bakker is a teacher and teachers like to teach. They also like to be cheeky about it. I figure he wants to show us in the 3rd series, that we humans control our own fate. He's not finished yet, so dont be so critical. And, as the man said hisself, what he did with TUC, well, ive never read nothing like that before in fantasy. And, i read a shit ton of fantasy, and its always the same, "they" are in peril and then find this miraculous way to win the day. Some is done well, others is shite.