That would mean that ALL 500 of the US' largest companies by market cap all go completely bankrupt WITHIN ONE CALENDAR YEAR.
I think if atomic bombs were to destroy the 50 largest cities in the USA, that this wouldn't happen. There would still be enough economic activity in enough of the 500 companies to justify some price above zero.
So the scenario would literally be the apocalypse. The kind of apocalypse where the US dollar has zero value. The kind of apocalypse where the amount of money in a brokerage account would not even be on the top 100 list of things a human would be worried about.
Even losing 90% of value would be an event so unprecedented in magnitude that it would make anthropomorphic climate change seem like a footnote in history.