They aren't dumb enough to just come out and seize your 401K but they can rather easily get a large portion of the populace to "voluntarily" hand them over by using the right incentives at the right time.
Just off the top of my head:
Market crashes (nothing doomsday, lets call it 50%
, people still haven't recovered from the last crash so now they are really down.
The .gov steps in to "help" and offers a program that gives you pre-crash value of your 401K with the catch being you have to move all of the money into their new "retirement plan" that has a guaranteed rate of return (basically instead of buying stock your buying .gov debt).
Throw in a few taxes to hurt 401Ks and a few tax breaks for the new .gov retirement plan and voila a whole slew of new money for the Federal Government to spend.
Social engineering, it is flat out retarded for them to "force" people to do something that will likely result in the pitchforks coming out when they can simply offer enough incentives to get you to willingly hand it over especially if the people lose even more faith in the market due to another crash and the bullshit computers fucking over people.