>With PoS you'd need to slash the validators stake on a fork; which isn't going to happen because the stakers run the validators everyone is using.

Sorry but I don't see your point here. A fork inherently requires action, it's enough to force a node to follow a different block from some height. At that point the attacker would get penalized for being offline. Outright deletion would require code modification. It's even possible to automate a minority fork in the case of censorship, although this capability doesn't exist yet.

>You already saw this with the steemit takeover.

Steem isn't even PoS, it's dPoS. Even in this case users successfully created a fork called Hive and removed Justin Sun's coins.