While explaining this theory elsewhere, I realized that while we play through the events of Ocarina of Time, we are basically in the Adult timeline. If this is all correct, Link's failure for the downfall timeline happens before we play. The child timeline is created after the final battle...
Ocarina of Time is an adult timeline game!
Okay, I think I get it now. While OoT is the start of all three branches, it's set primarily in the AT. Just like what we determined about the player never playing as the Link that dies to start the DT, they aren't in the CT either. Following a linear course of events in OoT, Link pulls the Master Sword, sleeps for 7 years, wakes up and beat Ganon. It's only after all of that Zelda sends him into an alternate timeline where that had never happened. Whenever Link puts the Master back in it's pedestal we know it's taking him back in time, but within the same timeline because the effects are still felt in the future (such as with Guru-Guru). But once Zelda sends him back permanently that the child and adult futures move independently of each other. Up until that point Link was simply going back and forth within one timeline- the timeline that would eventually lead into the events of WW. So while the events of OoT bring about all three branches, the player is specifically playing within the events of the AT. The rest are caused via interference- the CT being Zelda wanting Link to have a childhood and the DT possibly being the ALttP triforce wish.Look at it this way; the chain of events that lead up to Wind Waker, and the rest of the adult timeline, are as we see them in Ocarina of Time, including Link being sent away. The events that line up with Ocarina of Time, after Link was sent back, for the child timeline, was vastly different. We don't get to play those events. If the theory about Link having no chance for success, leading to the downfall, leading to an alteration in time, making it so Link can win, back in Ocarina of Time, is correct; we don't play those events either. The story we see, and play, are only part of the adult timeline.