There are no miners in a PoS chain, and PoS participants technically have nothing to lose through double forging. There are myriad different ways to find and punish the bad actors in such a situation. I don't claim to know about all of them, however they all make the same assumption you made that staked validators are rational people who look after their interest on the blockchain. But what if they are not rational, or worse, could make gains by disrupting the chain?

This is far from a solved problem, and this is the reason why PoS coins exist mostly on the fringe and ETH is taking very slow baby steps towards a ETH2. Nobody is certain that PoS will stand the test of time and my prediction is that it never will.

One might argue that they don't have to worry about anything judging from how well exchanges has been able to coordinate with the developers to pick a side whenever a major fork is spotted. But then it calls into the question whether we actually need a blockchain when a couple of interested party are the ones who gets to decide which fork is valid.

> So PoS is vaporware because it hasn't solved a problem where the miners can act against their economic best interests and mine a fork?

Not exactly. The point of "nothing as stake" attack is that you can pull off the attack while having... "nothing a stake".

Except the trust of the currency itself, as if you're attacking a currency you have a lot of stake in, you reduce the value of your stake because you attacked it.

I'm pretty sure short selling crypto is still a thing. Loss of stake is jut another cost of business.