I partially agree, yes. The world 's certainly has a very connected economy, so well drag each other down when the major countries drop out.Ok fine, if USA and China crash the world will crash. If you disagree with that you don't understand economics and where supplies are coming from. Like it or not your country is dependent of both.
But I doubt it'll come to a total crash, for this very reason. Companies that go bust will leave a void that can (and will) be filled by others at some point, as supply and demand will pick up again.
It's interesting to see how countries will overcome this. I've got to be honest : your 'oh, right... And China as well' isn't so casual as you make it out to be. China has had a long state oriented approach with years of economic growth that allowed them to build up reserves. America's recent economic good years were marked by tax cuts and stagnated worker loans.
To put things in perspective : I'm personally put on 'unemployment by force of majority '(overmacht) meaning Belgian government and my company both pay me about 70% of my loan. Taxes will be lower as well (and Belgium has a huge tax rate). On top of that there's talk that gas and electricity bills will be temporary paid for, and it's possible that mortgages will be temporary put on hold as well (though that's still uncertain). As a result, I might actually BENEFIT from this situation. That probably seems weird to Americans (yup... That's socialism for you
... which brings it back to that situation of shares economies. Again :we'll feel a big impact if companies or even countries go bankrupt... But I don't see that as likely if they haven't saved up for a situation like this.