I feel this "nothing at stake" argument is being used without proof of its existence. Even though theoretically it's possible, but it also goes against human nature and logic. Why would you attack and destroy the reputation/value of the currency, that you hold an extremely large stake in? it makes no sense to attack yourself and destroy your own wealth.
From a "normal person's" point of view I'd agree - but from a "government's point of view" things change (as they can just print their own money as long as they wish).
So if a government wants to "destroy" a PoS chain then they can just buy up (paying more and more as they print more and more) and they will succeed (the cost is always *zero* for them at least in terms of the "fiat").
In PoW even if a government wants to spend billions to kill of Bitcoin they just don't have the ASIC equipment to do so.