RE: What happens if you withdraw $1 million dollars from Coinbase into your bank?
Yes, but that's still cash which is locked in the crypto space. I can't imagine any real world bank viewing this as collateral, at least for another 5 - 10 years.
You ask why would anyone move crypto into fiat... isn't that obvious. The future isn't here yet.
Obviously, most of us don't like the current banking system but we have no choice if you want to spent your crypto millions. As much as we love crypto, it's not possible to live on it without fiat.
What if someone wanted to buy a house or a car with their crypto profits? They would need to cash out into fiat for the foreseeable future. That is the reality today.
I'm optimistic about cryptos future but right now it is a pain in the ass cashing out any large sums into fiat because all real world banks are regulated by anti-money laundering rules which make it incredibly hard for large withdrawals to happen. Esp in the UK and US