The money that is moved from a 401k to the market, so they can spend and save it as they wish. You don't have to spend all the money you make that doesn't end up in a 401k; in fact, if you do, you're doing it wrong.

The only people whose retirement would be adversely affected are mutual fund managers.

How about everyone who has to suddenly start paying taxes on the money they're saving for retirement? 401k is just a tax shield. You want to raise taxes to stimulate the economy??

You can't move money from a 401k to "the market", because 401k accounts are already a vehicle for market investment in the form of low-fee ETFs or mutual funds.