I reread Creating a Champion and in regards to timeline, it really wants to have its cake and eat it too. Referencing the 'good ending' of Ocarina:
"Ganondorf, king of the Gerudo, transforms into Dark Beast Garron and threatens Hyrule. The princess of Hyrule and the chosen hero combine their power to seal Ganon."
(could this also describe the events that led to Downfall? Maybe, but that doesn't seem to be the implication.)
But also a cycle of constant Ganon revivals, which lines up with Downfall better, and is unlikely to have happened in the Adult timeline as (a) Ganondorf himself is at the bottom of the sea with the Master Sword stabbed in his forehead (b) the Adult timeline currently ends in Spirit Tracks which is in New Hyrule and BotW's world is Old Hyrule
"In a seemingly endless cycle of darkness and light, Ganon continues to be revived and then sealed away."
However, there may be way for BotW to be in the Adult Timeline. Some dialogue from the Great Deku Tree in Wind Waker, suggesting a 'restoration' of Hyrule.
"
Every year after the Koroks perform this ceremony, they fly off to the distant islands on the sea and plant my seeds in the hopes that new forests will grow." β
Great Deku Tree (
The Wind Waker)
"
Forests hold great power: they can change one tiny island into a much larger island. Soon, a day will come when all the islands are one, connected by earth and grove. And the people who live on that great island will be able to join hands and, together, create a better world. Such is my dream." β
Great Deku Tree (
The Wind Waker)
Despite the founding of New Hyrule in Spirit Tracks, perhaps generations after once the islands of Old Hyrule are 'pulled together' and the sea levels changed, members of the Sheikah tribe returned and stared to form their advanced civilization, and there was an exodus of Hylians 'back to the homeland' so to say.
If one argues that this narratively goes against the ending of Wind Waker - I'm inclined to agree with you, though, it could still work in the sense that all these new islands coming together are still a 'rebirth' of Hyrule, and not strictly a restoration of the old one.
Eh, just an idea. Personally, I think process of elimination leads to Downfall. I just thought these quotes were interesting, as my memories of Wind Waker are foggy.