It's a difficult attack to be sure because owning that much stake in a network is unlikely - but it is absolutely not impossible because many PoS systems especially have very lopsided distributions. Losing the ability for new nodes to know what is the "one, true chain" without needing outside information is a problem. How big of a problem is a matter for debate, but it can't just be brushed off as so unlikely as to be impossible.
...and, indeed in the scope of the topic of this thread, it becomes much more problematic than just a contemporary majority stake holder turning bad, even recently emptied private keys can be used to carry off this attack as long as there is no objective way for the network to determine the true chain.
True chain can be determined by comparing block height with the block explorer for PoS or PoW.
As a Sybil attack can fake a chain on either PoS or PoW and only comparing to a Block Explorer can verify the true chain for a syncing node.