You are oversimplifying their argument. People are suggesting that people use more than one bank or make a service that involves using more than one bank not necessarily infinite banks. It isn't necessary to choose between zero risk and existential risk. You can with only modest effort achieve moderate risk.
the math is still the same though, just different degrees. If we (as tax payers) are ok with insuring 3 accounts (as an example) x 250k each, why are we not ok with insuring 1 account for 750k?
