1) There are no references to the Sheikah in the Downfall Timeline. Yes, it is assumed that Impa is always a Sheikah, but that isn't necessarily true.
As I mentioned in one of our previous debates, the fact that Impa was retconned as a Sheikah would mean that her appearence later in the Downfall Timeline confirms that tribe exists still, and she is part of it. Impa is always a Sheikah name in origin, and her role as Princess Zelda's attendant parallels that of past versions of the character chronologically. Basically, if Nintendo wants you to think of Impa in modern games as a Sheikah, the same must also be true of past versions of the character too.
2) The Temple of Time doesn't exist at all in the Downfall Timeline. It plays a key part in both the Child Timeline and Breath of the Wild
The Temple of Time actually does exist in the Downfall Timeline just as it exists in Twilight Princess: in ruins deep within the Lost Woods. This is not evidence indicitive of one timeline or the other.
3) While it is technically possible that Ganondorf could exist in the Downfall Timeline, he doesn't. We don't see him anywhere. We do, however, see him in Twilight Princess. Where he dies. Do you know where we see a Ganondorf with nearly the exact same jewel on his forehead? A dead Ganondorf? With a hand gripping the exact same spot where he was impaled twice? In the trailer for Breath of the Wild 2.
Every timeline has one thing in common, Ganondorf starts out as a Gerudo in some variation of the events of Ocarina of Time. As has already been pointed out, the numerous attacks of an inhuman form of Ganondorf only seem likely on the Downfall Timeline, nowhere else.
I'll give you that it looks like he has a hole in his chest similar to where he was impaled in Twilight Princess, but any evidence from Breath of the Wild 2, an unreleased game still in development, remains to be seen, and isn't substantial enough at this moment in time.
4) Nintendo might care about A Link to the Past, enough to make a sequel, and they might care enough about Link's Awakening to make a remake, but they haven't continued the story of the Downfall Timeline other than by sequels and remakes that play it pretty safe. It is the designated zone of 2D games, the ones that aren't as popular nowadays.
A Link Between Worlds was a very popular game, and since I obviously think Breath of the Wild is most likely to take place on the Downfall Timeline, I don't think Nintendo doesn't care about this section of the timeline. It has at least gotten 2 new games definitevely, whereas the Child Timeline hasn't gotten a new game since Twilight Princess, and that was 14 years ago.
5) The Downfall timeline should not exist. The Child and Adult timelines are based on a time split at the end of Ocarina of Time, but there is no reason for the Downfall timeline to exist. It is literally just an alternate history, which defeats the point of having an actual timeline in the first place. The Downfall timeline is the place where Nintendo put the games they didn't think fit into the main storyline branching from Ocarina of Time.
I'm going to have to agree with Moblinking here, it exists whether you want it to or not. Plus, this timeline is actually one of the more well developed ones, even if it's existence was put together as a cop out at the start of the previous decade. It makes sense though, and is a better explanation than anything I've ever seen before it, I think it's pretty intuitive and not at all impossible, because ultimately this is a fictional world which already established that multiple timelines are a possibility long before Hyrule Historia was released.
6) Breath of the Wild Zora's Domain is in the same place it always has been: Lanayru province, southeast of Death Mountain. Whether or not they rebuilt it 10,000 years ago doesn't matter. Even if, for a time, the Zora did not live in the Lanayru region, that doesn't mean that they left Hyrule. There's no connection between those two things. They could have had a different home. 10,000 years is more than enough time to move the hub of your civilization around several times without leaving the kingdom.
I still think geography isn't a good argument to bring up in a series that always moves places around, and might do so again in the distant future. But I will point out that the Lanayru Province in Twilight Princess consisted of Zora's Domain in the northeast, and Hyrule Castle in the center of the overall map, slightly southwest from the domain (the Gamecube orientation of the land is the canon one, by the way). In Breath of the Wild, the Lanayru Province is situated firmly in the east, Hyrule Castle is in it's own province designated "Central Hyrule" and there is no evidence of any water bodies that had lakes to the northeast of the castle. Instead, Death Mountain is now in the northeast, but in Twilight Princess it was almost directly east of the Castle... I think we can agree this doesn't match Twilight Princess at all. In a more broad sense we could say it matches Ocarina of Time's Hyrule if it was a lot bigger, but closer examination rules that out too if we take the monuments in Zora's Domain at face value that claim it isn't the original Domain. Additionally, Kakariko Village is nowhere near the foot of Death Mountain, plus the Temple of Time isn't in the same vicinity as Hyrule Castle.
Bottom line, geography in Zelda is wholly pointless.
Does to me. Calamity Ganon is revived time and time again, and after each period of decline there is a Golden Age. Sounds very much like the Child Timeline and Breath of the Wild are in the same timeline.
There is no indication of the Child Timeline having numerous attacks from Ganon, it simply doesn't have the history laid out for it, claiming it does is nothing short of fan-fiction with the games the Child Timeline has.