So instead a cartel of miners you'll get a cartel of crypto-banks and exchanges with lots of coins in their cold wallets for which they'll additionally earn interest, without having to reinvest in new generation hardware. MtGox would have dominated a PoS-version of Bitcoin quite exclusively back then.

Not that I know of any better solution, concentration of wealth and power seems to be a general problem inherent in capitalism, or possibly even any imaginable form of human society.