The Adult Timeline is most valid, in my opinion. The Downfall Timeline is the "What if" scenario (What if Link fails). And the Child Timeline is an alternate history. The Adult Timeline progresses exactly the same as it would have with the only exception being the Hero of Time is no longer present. Him not getting sent back in time by Zelda wouldn't have changed anything for the Adult Timeline. It is implied that Ganon broke out of the sages seal at some point after what would have been the Hero of Time's lifetime.