I can only speak about Bitcoin here as I'm not as familiar with how other coins work. Bitcoin is basically designed to make this hard. In order to spend money

I can only speak about Bitcoin here as I'm not as familiar with how other coins work. Bitcoin is basically designed to make this hard. 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.