51% attack by non-financially motivated party

I've read a lot about how POW and POS both protect the network in various ways. But most of these factors depend on the attacker being financially motivated.

What happens when the attacker is not motivated financially, and simply wants to disrupt the network? What if they are willing to pay the price to do so?

One example would be large institutions (government or central banks) wanting to disrupt the network to promote their own "better" network. They probably would not care about spending billions to acheive their goals.

I have given this problem some thought and one argument for POW would be that it would be physically impossible to gain enough hardware in a short period of time to achieve this. So we'd at least see it coming and have time to respond.

However this would not be the case for POS (I think), since you can simply keep purchasing on the market even if it drives up the price, obviously you would still have to spread your purchases out at least a little bit.

I'd like to hear some arguments to counter this.