I never got into Preacher though I read the first trade. Y the Last Man does have a wonderful ending. I was spoiled about 10 issues from the finale with a basic description and thought that I would hate it.
The thing that makes Y's ending so good is that all the stories before it add to the ending. I doubt that a TV show could be scripted that tightly. Producers often try to keep the series going for a set period of time and than stop. The only way it make sure the series would work is if it was a direct adaption, episode for issue. And if you did that, why bother?