MSQ Speculation

[Lunaxia]

I just can't see starting an expansion story from only a letter we see for less than a patch as a viable thing to do. Cutting off all the other threads just wouldn't make sense.

The brutal truth is, as far as the devs are concerned, Endwalker is done, and I mean done. They've made a very deliberate point of hastily wrapping up just about every lingering thread left from the previous storyline and tossed the entire arc in a box they're clearly determined to leave on a shelf for now. It doesn't make a whole lot of sense, but that's what they've been doing. Ordinarily, we would have something like you suggested running alongside the Void story, building off past plot points and leading us into the next expansion, but they've decided on a different direction this time around and seem intent on emphasising DT as the start of a new chapter. Running back to Amon and the Allagans would go directly against that.

I'm sick of it too.

I think we're meant to assume the Ascians have been pretty much everywhere at some point, whispering in ears behind the scenes and stirring up strife to help set up the events that triggered the various Calamities that transpired over the ages. I don't know if they need to make a point of highlighting their involvement with every civilisation on the planet, and it would start to get kind of obnoxious if they did.