As for the developers, they could write code to print a billion bitcoins if they wanted to, but for it to take effect everyone's gotta agree to run it.
Holdouts will find that old-coin transactions never succeed, and will have no other choice except to walk away in financial ruin.
1) Removing the attacker directly (PoS) / changing the hashing algorithm to remove the attacker (PoW)
or
2) Submitting to the whims of the attacker
it would be a no-brainer to just remove the attacker. Embracing an attacker-controlled chain seems like a strictly worse option than removing the attacker, no matter how messy. It seems like a choice between disruption or certain ruin.
An attacker at that scale would certainly understand this game theory, and realize that if they attack, after some potentially very messy shake-out and serious temporary disruption, they will soon have no influence over the chain anymore.