That's only possible once - from asic PoW to a gpu/cpu PoW.
After that, if the fork becomes popular, the attacker moves and attacks it again. Are people supposed to fork once per week? At that point it's not PoW consensus anymore, because the interventions move from emergency to normal, and you're under something else.
It's not just miners, it's also full nodes that validate.
Full nodes can disagree with miners: instant fork.
If say Putin would acquire 51% of mining hash rate people would realize (before he gets there) and would fork.
My comment is not in support or against PoW, I'm just stating the fact that PoW isn't inherently unsafer than PoS.