there's not nearly that much cash in those accounts, because something like 90% of all 401k/IRA money is invested in stock, mostly mutual funds. Probably even higher if you don't include retirees.
What I haven't seen yet is an explanation for *how* the 401ks could be confiscated, in a way that wouldn't absolutely kill the stock market. If they wanted to raise money by confiscating the stock and selling it off, the market would collapse. If the idea would be to confiscate and hold the stock, and use the resulting gigantic government-owned index fund to back up the annuities that would be created, then that doesn't raise much cash.