In a fiat money system, money that is in a mattress does not exist.
The percent of the money supply in cash is miniscule. Do some research. Even if ALL the cash was hoarded it is so small an amount as to be a non factor. In defense of you r position: The only way your scenario would be viable is if/when their was a run on the banks and the fed had to actually crank up their printing presses. Rumor has it the Fed has printed the money for a run already. I think the Feds want a run but that is my paranoia coming out. It could happen, but it isn’t right now, so you scenario is moot.