The alternative to a billionaire deciding how to spend the money they have earned is a politician deciding to spend the money they didn't earn.
I trust a person who has generated billions of dollars worth of value over a buerocrat who won a popularity contest.
Besides, I don't think it should matter whether or not these people spend their money optimally. They've made this money, it's theirs. Don't take other people's stuff.
> We’ve already seen what more extreme individuals are prepared to do with that power.
Until we leave in a featureless dystopia where all people are perfect clones brought up in an identical environment, there will be power disparity between people. It can be based on inheritance (like in monarchies), or ability to commit violence, or charisma, or political skill, or ability to create value.
Between all these options, I'd much rather live in society where people get to keep the value they have created. It seems to me the most correlated with merit (not perfectly, but better than the alternatives), besides, it is morally right.