They could realistically change the tax structure or even the retirement age(s) at which you could withdraw the funds. But if there were another mega-stressor to the financial system in the future, it seems that this pool of trillions of dollars would be ripe for the picking. Let's face it, lobbyists get what they want. Not the people. So if the bank's risky investments cause a panic and the only thing preventing an economic meltdown is the re-purposing of 401k funds....well...