A blockchain is, fundamentally, a consensus mechanism which decides which transactions are valid.
Proof of Stake is self-referential; the parties with value at stake have value because the blockchain says they do. It's a perpetual motion machine. When the shit hits the fan (say, a controversial fork) the people who thought they had control ("stake") will learn who really makes the decisions WRT which blockchain is canonical. That would be the exchanges, and what they agree on will rule no matter who stakes what.
The reason Proof of Work works is because someone has to pony up the compute power.
> Well of course they have to determine it themselves otherwise they are centralized and dependent on a third-party.
PoW doesn't have that problem.
> Please expand on what you think is flawed and what are the downsides.
If I make a parallel version of the chain where I got all the mining rewards from block one and ended up with a majority stake, how do you decide between that and the other chain?
