Doesn’t matter that much whether it’s “100% backed” or whatever. The time it takes to (1) send USDC to Coinbase, (2) convert that USDC to USD, and (3) withdraw the USD to a bank account is short enough (less than a day by wire transfer) that the counterparty risk is pretty darn minimal.
Until it's not. Part 1 and 2 of your list require Coinbase's implicit consent and cooperation, and if there is a bank run on USDT, that cooperation may be in short supply.
I would bet money that Coinbase has a button on their admin panel that says "Pause redemption of USDC" or equivalent, so you're just hoping they don't push that button in extremis. I find that silly, if what we're hypothetically betting on is that the crypto system will be in extremis.
Also Coinbase is not so pristine or pure. They were misleading customers with a gentler version of the same thing Tether did. "Coinbase Vowed Token’s All-Cash Backing; That’s Not True". https://www.bloomberg.com/news/articles/2021-08-11/coinbase-...