I think it will simply become impossible to keep digital payment systems secure. The whole online economy will collapse. Amazon will become worthless. People will actually have to go out to local stores to buy everything. People will have to go to their local bank or currency exchange to cash their paychecks. The credit card system will fail completely too, while payday loans will take their place. The stock market will have to go back to traders yelling their trades to each other at exchanges. The old ways will become new.
We already have a system subject to impersonation attacks and simple fraud. Major retailers and fiscal institutions both spend a considerable amount of time tracking fraud, and often catch it so easily that they need to wait until it accumulates to a point where it’s worth reporting.
And, even if we were suddenly inundated with a level of fraud attempts sufficient to overwhelm the system, there is literally an entirely separate model of online finance that was designed to be zero trust.
And if “how existing banking” and “cryptocurrency” don’t convince you we won’t be reverting to cash, please realize that an AI ubiquitous to overwhelm global finance is also so ubiquitous that we might as well do what humans always actually do when commerce breaks down, and revert.to a gift economy.