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.
At most it would mean some things like some easy payment methods would be ended and it would be more 2fa and secure methods. The type of thing that is vulnerable is simple trust based things like calling for ordering food and appointments and stuff that can be done without payment before. That can be spammed.