It's not entirely clear to me what you mean by "regime", but it seems most likely to me that if forced with a choice of two hard-forks:

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.