In fairness to the movie, it is based on a book. It is my understanding that the movie is pretty faithful to the book, so any complaint with how anticlimactic the ending is may be the fault of the book.
I don't really mind the end. When you say the last 20 minutes, are you talking about every single part of it, or what? There was only one part that I didn't really get at first, which was
why James Brolin was just dead all of the sudden. I read something online about how we are supposed to see the whole thing from Tommy Lee Jones' point of view, because in a way this is his story, so getting there too late fits that. I'm still not even convinced that's correct, but I think it's close because in a way Tommy Lee Jones is the most important character.