I want to talk about tempering

[Vulcwen]

But... if vanquishing a primal returns its aether to the land, then something like the Burn couldn't come about as a consequence of summons... right?

Well, it could be that the aether moved to other parts of the world. A land drained of aether can't support the lives of a primal's followers, meaning they'd likely have to move to other places, and the primal would move as well.