Earlier quoted context omitted.
Multiple parties need to fail for USDC to not be tradeable or redeemable to USD - holding in an exchange just requires one party to fail (exchange hacked, exit scam etc.) It's pretty simple risk logic.
If the issuer of USDC fails to redeem USDC with USD, the price of USDC in the market will collapse, so I'm not sure which "multiple parties" have to fail for USDC not to be redeemable. Further, keeping funds in a non-custodial wallet entails a huge operational risk that you're glossing over entirely.
The OP was about the safety of "holding" fiat on a single exchange vs USDC on your wallet.
Your point about non-custodial wallet risk is a misonemer as this is purely about can you trust yourself more than trust an exchange.