Or else they will boil the frog slowly by requiring that 401k administrators invest a mere 10% of the accounts' money in safe investments like government bonds rather than risking people's retirements in the dangerous stock market. Expect that type of argument to be especially powerful if the market drops 20% quickly. And if all the 401k accounts sell off their stocks quickly the warnings of danger of the stock market will be self fulfilling. Then that will be bumped up to 20% or 30% because the 10% requirement was so successful. Pretty soon 90-100% is in bonds with no one but a few "crackpots" screaming and no one shooting anyone.
In the end, many people will stop contributing new money to 401ks because they will get more return in non-retirement accounts even if they have to pay taxes immediately rather than defer them to retirement and many will take their 401k balances as cash with the immediate taxes and 10% (or more later) penalties rather than rolling them over into IRAs or new 401ks.
I could certainly see President Warren implementing that via Executive Order.