In order to spend money you need the private keys. No way around it, even with 51%. What you can do with 51% processing power is rewrite history. You start a parallel chain where the oligarchs never received their money. When your chain accumulates more processing power it will in theory be accepted as the canonical chain. Now there are a few problems here. First the simpler one: If you want to rewrite any significant amount of history, say undo a transaction from a year ago, you will need more than 51% to achieve it in a reasonable time. Think of two runners, one is a few percent faster, the other has a year long headstart. It will take very long to overtake. With 66% of computing power you can do it in a year*. Renting that much power for that long year would be extremely expensive. Second problem. It's highly doubtful that people would just quitely accept such a huge rewrite.

* Assume the rest of the chain has processing speed X. If you have speed 2X then it works out to 66%. One year of 2X power equals two years of X power, catching up with the 1 year headstart.