Long anime series are treated a bit differently than North American live action productions. One Piece will most likely continue being made as long as there's manga to adapt. (Whether the mangaka will finish the story, drop it abruptly to do something else, or die of old age with it still incomplete is another question.) The only other thing that might kill it is if the studio making it folds, but as far as I know Toei is relatively healthy.

At ~1170 episodes, One Piece isn't even close to being the longest-running anime series airing in Japan today. I don't think it's even in the top five.