My take is that this is basically what inflation is. Use the money for something useful, or it becomes worthless.

This is basically the principal behind universal basic income; adding more money to the pool by sending everyone a monthly check devalues the existing supply of money. If you have a billion dollars stuck in a savings account, the purchasing power of that billion goes down, incentivizing you to use it on something useful. That hurts you more the more money you have. As for prices increasing because of inflation; that's the beauty of mailing everyone a check. That takes care of that problem at the level of basic needs (food, medical care, housing, transportation), but not at the level of private jet flights being subsidized.

Rich Americans would move elsewhere and renounce their citizenship probably to avoid that tax %.

The effective tax rate for the rich has stayed very flat over time, and it has never been 90%. That is an internet myth that is somehow floated around that most people don't bother to check up on.