Sorry you do not understand how the system works. You can't buy 50% because the price would be worth more than all world gdp since each order is incrementally how gher and the supply of coins is not unlimited. Good to dig into things before you mindlessly dismiss them. Did you actually believe that people are designing and talking about Pos as a breakthrough when the main idea is "nobody rich enough will want to buy 50%"?
For newer PoS protocols, they would need over 66% and they would lose money if they misbehaved making it likely uneconomical to do so. This is the big advantage of PoS over PoW. There are both positive incentives (e.g. block rewards) but also negative ones (e.g. getting slashed for trying to stage a double spend attack).
