How do you envision the mre robust economy?

Hoarding on its own shouldn't be a problem with fiat currencies. The Fed keeps interest rates low until there is some inflation.

If billionaires keep money in a bank, the bank lents that money to those who spend it.

If money should be spent more wisely, everybody could spend their income each month more wisely.

So, what exactly do you want the billionaires to do?