I just finished rereading the series recently and, while yes I do agree that the ending is unsatisfying, but I can pretend I don't still find it to be the perfect ending for the setup. Roland's on a journey to (whether he wants to or not) save the Tower. The Tower, meanwhile, seems intent on forcing him to save himself in the process. He'll keep doing it over and over until he essentially gets the perfect run. Where his friends survive, he doesn't lose the relics of Eld along the way, and (most importantly) where he doesn't need to sacrifice his own son to see the quest through to the end.
That's my take on it, but even without that, you are thoroughly warned to accept the final chapter and epilogue before the final final chapter as the ending. King flat out tells you not to keep reading past that... I doubt it stopped any of us, of course, but the warning is there. And without that final final chapter, the ending we get is significantly more satisfying. Roland completes his quest, the Ka-Tet gets a happy(enough) ending, and the multiverse is safe forever and whatnot... Good ending. Not perfect, but way more satisfying than a lot of King endings.