First, how do we know Demise created that same fissure, instead of it being named after him and his legend by the inhabitants of Hyrule? We literally have no connections other than the name, and several places in BotW bear the name of characters in omage to them.
Because it's a very clearly unnatural formation that is consistent with the lore. The locations in game while homages, aren't always the case, something like "Tingel island" is an homage, something like Spectacle Rock though or Death Mountain, is very clearly the same location from Zelda 1, or how the Temple of Time is clearly the same Temple of Time from OOT, almost explicitly so.
Or that one ruins being the Temple from Skyward, etc. While some places are merely homages, that isn't always the case, some places are named a way because they're literally that place or a place referenced or seen in past titles.
Between the time Demise created the original fissure and the events BotW thousands upon thousands, if not millions of years have passed, there's literally no way to tell the structure of the fissure has remained the same, with all the possibly geological changes that could have occurred in such a large timespan.
Probably only a few ten thousand at most, millions is extremely pushing it.
While it's true it could've changed, it's also true it probably hadn't. A giant fissure that split the earth isn't exactly something that just ups and vanishes, it'd take millions of years for a geographical scar like that to change.
The place itself isn't even a proper fissure, it's more like a canyon, which is way different than a fissure in the ground caused by something like an earthquake.
Ok and? Nobody said Demise created an earthquake. In fact it's simply said the ground suddenly split open and he rushed forth from within it, for the most part a canyon being created due to that split makes sense. It being a fissure is a ENG only translation statement fyi, Japanese merely says the earth split open suddenly.
I might be missing something, but how can this thing be calculated and used a feat, with all the uncertainties and the fact we are not even sure the place is the same?
It's very clearly intended to be the same place. it's not even named after him like "Demise canyon" or "Canyon of Demise" or "Demise Fissure", etc, or whatever, it's literally called "The Breach of Demise", it's honestly one of the only places in the whole game named even remotely in a vain like that. Most are just "name island/forest/rock/mountain/lake/etc", no other location describes a location's name in the same way. Calculating it is easy, we know it's described as sudden. Just calculate it as a divide. Hell the fact it's called the
breach should be evidence enough it's not just a cheeky reference, but rather actually referring to that time Demise breached the surface world and created a split in the earth.
Honestly, while I get the caution, it's pretty unreasonable, it's clearly intended to be the place Demise came forth, it has a unique naming convention, has a completely unique structure not seen anywhere else in the whole game. The worst part is it ain't even a good feat, he'd scale to Tier 6 off Hylia and this feat wouldn't get past 7-A at best.
