If the government has de facto control of the accounts, they can just issue 7.8 trillion in new debt backed up by the value of the accounts - they don't have to actually liquidate anything.
If they were to do that, it would destroy the market capitalization of every corporation whose stock is held by those retirement accounts. And it would instantaneously convert the USA into a communist country.
Of course, those are the wet-dreams of most of the leftists in government today.
Mark