Slashing has almost no effect on nothing at stake solution as majority determines what gets slashed and what isn’t due to trivial censorship by that same majority. And funniest of all, slashing for censorship is actually trivially abused by the majority for profit since they end up with more during mutual slashes. Since slashing increase scarcity of coins and their value, there’s incentive to form cartels so at high enough percentages they minimize slashing damage. This is why most proof of stake uses checkpoints and relies on honest majority, including Casper.

In proof of work such majority would be continuously paying costs for electricity, lose money by not covering their hardware sunk costs, and eventually run out of ability to attack.

Slashing is the worst possible design for any blockchain as it’s trivial to abuse for a majority and in the end the cost is limited to the amount of stake holders have in either PoW or PoS as economic majority loses confidence.

Incentives from the economic majority makes block chains secure.

Otherwise miners could profitably form cartels and double spend without end if the value of coins didn’t degrade without any need for something as irrational as slashing rules.