P.S. Here is a realistic risk of PoS: a lot of people have proven to see cryptocurrency as a trading tool and they will deposit their coins on a centralized exchange. All of a sudden we have one entity with a huge amount of coins aka stake that can control the PoS too.
And it already happened several times. For example, Binance, Houbi and Poliniex helped Tron foundation to perform hostile takeover on Steem network[1]. Binance only apologize[2] while Houbi make an excuse[3] without any real action.