Sorry, but that same paradox exists in all single time line time travel movies, such as BTTF and the Terminators. If by changing the future Marty no longer exists, how can he come back and change the future?
It's a time loop - hence the title of the movie. You can't change the past, it's already happened. Only events from that moment forward can be altered. Young Joe kills himself, old Joe ceases to exist from that moment forward. yes, it means that when the time comes in the future where he would have traveled back, he doesn't...but that's the point.
They actually did an excellent job explaining this point by showing you the two very different returns of old Joe - the second time (or is it the millionth time) he comes back, the outcome is different. The future is not set went the timeline is altered, only the events before the alteration.