10 billion?
I can't see how someone realistically would needs more than 10 million, but okay, let's set the limit to 500 million. A ridiculous amount of money.
But 10 billion? That's an unspendable amount.
(And to think we will see trillionaires (1,000,000x million) in our lifetime...)
Right, I would say the limit should be "no billionaires" so that if your net worth including investments exceeds $999M then you have to sell off some property or give money to the taxman until you are under $1b net worth. $999 million is still more money than anyone would ever need but just having no billionaires would be a great start.
I'd say make a progressive scale such that 1 billion is the theoretical limit, meaning you would need infinite gross/pre-tax wealth to exceed 1 billion in net wealth after taxes.