6 books building up the existential evil that lived at the center of all existence, and when he gets to the tower to face the evil it’s just an old guy on a balcony throwing Harry Potter hand grenades.

Funny, I absolutely loved this. The banality of evil. And good, too. Everything. The world is falling apart. Even the great evil is not, in the end, that great. Two old (REALLY old) men at the ragged ends of their lives trying to do this one last thing.

through thousands of pages, and it's just a sad, pathetic, uninspired, lazy ending.

I mean, it does literally warn you to stop reading when the characters other than Roland get their happy ending, so if you kept going that's on you... /s

Also, it's thematic to the story at hand. It also ends hopefully, as Roland has the horn he did not have the first time through, which is implied to be incredibly important to his quest going well. We see the cycle right before victory, when he gets everyone else their happy endings and redeems his sins enough to earn the horn and, on the next cycle, likely end his quest. Which can be read as very hopeful, but your take isn't invalid or anything