> I wanted to steal Satoshi's coins. Could I invest a half billion dollars and brute force a majority-coup attack on the blockchain?

No - controlling a majority of the compute doesn't allow you to steal some unrelated users bitcoins.

You'd have to brute force their private key to do that, which is an infeasible computational challenge.

Controlling a majority of the compute would allow you to 'double spend' coins you have, but that's a different problem.