Well slap me and call me Sally. This time boomhauer is more optimistic than I am
I concur with Boomhauer. Outright seizure would be illegal (not that it'd stop the feds from trying), but more importantly it'd piss off every bank, most companies and 88,705,000 voters.
Banks happen to like having about 4 trillion in assets that the owners of said assets cannot move or withdraw easily.
Companies like having 4 trillion dollars put towards purchasing their stock.
Near 90 million people are putting money towards retirement. They're probably more likely to actually be voters, but I haven't seen any statistics. Stealing money from 28% of the population won't go over well. 37% of the over-18 population. You'd really need to sell the other 63% in this case. A very large percent of that 37% would be willing to put campaign contributions and votes towards revenge candidates.
So yeah. Screwing around with the tax implications is possible, even probable. Out and out seizure? Not likely. There was significant push back against ACA/Obamacare, which was mostly screwing around with existing structures and not nationalization.