Oh boy, time for my favourite crackhead theory!

Endwalker Spoilers ahead

Dancers have been harnessing dynamis all along, and the stuff they've been fighting for millenia to keep in check are the echoes of the song of oblivion that manage to pierce through the celestial currents. Thavnair got hit by the Final Days first because the currents are weakest there, which is why dancers originated there as well. That's why dancing is so esoteric and doesn't have its mechanics explained from an aetherological standpoint unlike most other jobs- because the mechanics that drive their capabilities aren't understood well enough in-universe to be elaborated upon. Bards also utilize dynamis, and possibly dark knights as well.