Which means infinitely better security than PoW. PoS is resistant to nothing at stake attack as long as at least one smallest stake is indefinitely honest in each block. It doesn't have to be the same, just so that there's no way for an attacker to achieve 100% at any one point. That's because the stakes of true owners have the same voting power, so they cancel.
To not cancel, the attacker has to own something in all histories, but now the cost is real. Even 0.1% of bitcoins is worth more than what ~17 days of mining power costs. He needs to own more than minimum stake of honest parties over all blocks.
The practicality of that is another matter, as is quality of existing implementations, but that's a much much more realistic requirement than using >half of all worldwide available energy for PoW.
If someone still thinks PoW is more secure, consider this: two countries are at war. One uses PoW currency, another one PoS, owned overwhelmingly by citizens from its creation. Which currency system gets destroyed by the enemy country, wrecking their economy as a result? Related point: which one is likely to be wealthier?
P.S. I think in practice all cryptocurrencies are driven purely by social consensus, with pow/pos/other as false rationalization for it. If bitcoin had serious problems due to the current pow algorithm (like: China nationalizes Chinese miners and starts enforcing some regulation worldwide, eg. only state-registered addresses allowed) users would fork fast, thus proving it's not really PoW. Same with PoS, but it's better as it avoids waste.