Eg:
POW 99 ==> POS 100 ==> POW 101
Even if i came up with a new POS 100A block later, i will need to perform more work than POW101 to attack the chain.
But if I am the generator of POS 100, I can also be the generator of POS 102 and the probability that I will be is a constant.
This is no different than saying that a large Chinese mining pool can withold blocks and slow the network to a crawl.
Not quite, unless you intend to award POS miners with a block reward? POW miners have a lot to lose by doing this, POS miners have nothing to lose.