It's going to be really hard to tank a country such as USA like so. There's no other country in the world with the systems in place to handle what USA would release if it went under. If the USA started to tank then other countries would help prop it up. Forget understanding the economical fallout that would occur, you would see MUCH more hostility between countries.
USA is one empire that won't topple anytime soon. Everyone's too invested in it.
The USA is a the personification of "you need me more than I need you". So who will flinch first?
There are two key pillars that the US provides to global stability: bonds and the nuclear umbrella.
Bonds are the harder nut to crack if we wanted to create a post-American world that wasn't a total shithole (due to the utter collapse of global credit, stocks, and a long-ass depression). The best route forward right now seems to be about giving more power to the IMF and their "special drawing rights," which would be based upon a basket of currencies rather than relying on the dollar and the US government, and could serve as a stable asset, but they only would work as a stable sovereign asset and nobody else is really there to pick up private investors who want a rock-solid investment (the European Union, if they got their shit together, would be a good bet, but that's not going well).
The nuclear umbrella, unfortunately, could be resolved by greater proliferation as Trump himself has suggested. Germany, Japan, and South Korea could be there by July if they decided today that they wanted nukes of their own. Saudi Arabia could probably get Pakistan to sell 'em some.