I think the author didn't want it to end, and in a way Roland didn't want his quest to end either. Thats all he new, searching for the Dark Tower. So it is what it is. Was I dissapointed, well let down is more like it? At first yes, and then it dawned on me that there could be no other ending for Roland the last Gunslinger or Stephen King.
Everything in between makes up for any lingering feelings of wanting a more......common ending. I loved the story and even the end. I guess if we were to all have a chance at a climbing to the top of the dark tower we'd all find something different anyway.