Examples of characters we'd like high level PCs to be comparable to

[Tanarii]

Its similar to how the powers of Good, Evil, Law, and Chaos balance each other out in FR. Yeah, each side has a lot of power on their own, but the second they try to wield what power, the other three are probably gonna join forces to keep the status quo. Thus creating a stalemate where those powers can exist in a coherent setting. Of course, once the stalemate ends then all hell will break loose, but that has the making of a campaign right there.

Depends. A post-apocalyptic wasteland is coherent, sure. Dark Sun, post-cataclysm Dragonlance, or Lord of the Rings are all good examples of why that's inevitable under the "balanced high powers" model, and how it can still be coherent afterwards.

But if you try it without the inevitable destruction, you end up with Forgotten Realms or Mystara. Totally incoherent. Still fun tho, if you can put it aside.