I think what could happen in a very bad and deep crash are: riots (EBTs cut off), panic, empty grocery shelves in some areas, and possible loss of essential services in some way (power, etc).
Even localized, these crisises will get international media attention, and that can make the problem worsen or incite the panic even more.
I would have some food and water on hand anyway even if there were not some very deep concerns about the global economy. It is a wise thing to do.
Well, sure. Having a few days of supplies on hand is simple prudence. We got iced in for a week here in North Texas a couple of years ago and the grocery stores were looking really sparse after the first 3 days.