I guess 51% attack is also possible on POS coins but it is easier on POW coins, PoS makes it simpler to penalize explicit actors in a 51% attack situation I think that is why no one does that on POS coins and they will be caught and punish specifically. This is on the grounds that PoS validators have moderately long-lived identities and PoW miners are changing their identities with each block and it is harder to trace them specifically.
I think PoS could be attacked if I can secretly print fiat currencies without limit, for buying up PoS coins and at the same time preventing the coins from moving up in price.
How about if most people mines on centralized platforms?
And I have to have certain amount of coin in order to stake and participate in the network consensus.
Almost everyone can afford a phone and laptop for minimal PoW or something, but not every can afford the expensive amount for participating in PoS.. This alone should disqualify it. A reasonable community should make it possible for everyone to participate and still keep things very safe. Besides, phones and laptops are multiple purpose aside mining or participating in network consensus... this is really good thing.
PoS is also very vulnerable in my opinion.