They can attack the network based on what they had (but no longer do) in the past. There is nothing at risk and no cost to the attack. THAT is the PoS problem.
Okay, I got that. However, calling it a problem is a rather bold claim. I would call it a property of PoS.
(No matter if PoS or PoW)
How would want to be on that fork anyway? In doing so, they would destroy every single bit of confidence in that very cryptocurrency.