Elon didn't make his fortune in a vacuum. (Well, his rockets might operate in a vacuum, but you know what I mean.)
If someone succeeds in a country like America, it is because they have benefited from the roads and schools and legal system etc. that the government has put in place, paid for by taxation. If Elon is more wealthy than most people, it means he has benefited more from these services than them, and thus it is natural to expect him to pay more.
The question then becomes "How does society determine how much to bill Elon for?", and there are valid reasons to propose taxes based on income, or wealth, or consumption, or land, or some combination, or some alternative ways of generating revenue, such as only taxing corporate profit.
Ultimately these practical questions are for the people and their elected representatives to answer, but if the answer is "Money in your bank account is taxed, but money in your non-profit charity's account isn't", then Elon is welcome to jump through that hoop to reduce the money that goes to the government.
Changing the rules after the fact may work on current-Elon (what can he do?!) but will definitely discourage future-Elons. And the society will benefit much more from those future Elons than from fleecing the current one to appease non-productive "equality seekers".
How many potential future Elons would give up on their efforts to "benefit society" if, instead of being able to amass a net worth of $270 billion, they could only reach, say, $135 billion because of a wealth tax? It seems like you're saying there would be 50% as many future Elons if an Elon could only earn half as much, but maybe you have some other calculation in mind.
Anyway, plenty of other people manage to be motivated enough to benefit society without ever earning a single billion (and some people manage to earn or inherit billions without having a net positive contribution to society whatsoever).
If someone succeeds in a country like America, it is because they have benefited from the roads and schools and legal system etc. that the government has put in place, paid for by taxation. If Elon is more wealthy than most people, it means he has benefited more from these services than them, and thus it is natural to expect him to pay more.
The question then becomes "How does society determine how much to bill Elon for?", and there are valid reasons to propose taxes based on income, or wealth, or consumption, or land, or some combination, or some alternative ways of generating revenue, such as only taxing corporate profit.
Ultimately these practical questions are for the people and their elected representatives to answer, but if the answer is "Money in your bank account is taxed, but money in your non-profit charity's account isn't", then Elon is welcome to jump through that hoop to reduce the money that goes to the government.