Let's be clear, Elon (or any billionaire) does not have billions of dollars sitting in the bank. He has assets valued at billions of dollars, mostly stock in companies or actual companies that he owns (like X). His wealth actually fluctuates with the value of those assets. If Tesla or SpaceX crashed, he'd drop down the billionaire list significantly. So he's not hoarding money here, he's accumulating wealth through increases in the value of those assets. If he sold off those assets, then he would have billions in the bank but people like him rarely do that. If they sell assets, it's to acquire other assets. So I am not sure what he is hoarding. If his assets go up in value, he gets rich. If they go down, he doesn't. He isn't hoarding anything. And if he sells those assets to give away the money, then someone else buys them and gets rich off of them instead, potentially. If you want to get rid of billionaires, then the solution is to basically shut down the entire market system of capitalism and go back to small, family or individually owned companies that operate entirely locally with no such thing as public trading of shares. That comes with a raft of problems of its own.
So I think we are going to be living with billionaires for a long time to come and the best approach is to structure tax systems to draw more of that wealth into public institutions. They are not going away.