How does the delivery and return of my USD work?
At every junction the answer is "well you trust another party and..." - so why is ethereum needed in the process at all? All of this is just companies providing an API for a service (which generally they don't, now, because it's complicated to orchestrate such a thing).
Ethereum is not contributing anything useful here, because everything important is "trust this person" - at which point I'm really just "hiring a lawyer".
The GP is describing a system kind of like Zipcar - you go online, you enter payment, and Zipcar allows your card to operate a car for a period of time. So, you might say, so what, we already have a Zipcar, why replace it with the blockchain?
Because if Zipcar goes out of business, all the cards and hardware on Zipcar cars are defunct. If all the hardware talks to contracts on Ethereum, there's no Zipcar to go out of business - you buy the hardware once, but the operation of the system happens on Ethereum, not in Zipcars' SQL database.
This still depends on trust. The blockchain doesn’t verify the backing. And USDC’s attestation is rather late.
There are stablecoins backed by USD reserves, like USDC. Again, the ethereum contract is only a ledger. The people responsible for giving you USD is going to be Gemini who control the supply of USDC. You having proof that you own USDC allows you to withdraw USD.
There are of course stablecoins that are not backed by USD, but rather backed by ETH directly and the USD peg is maintained through arbitrage, oracles and control systems implemented in the contracts.