The thing is, "not your keys, not your coins" is not just true technically, but true legally. If Coinbase were to become insolvent and decide to use your coins

The thing is, "not your keys, not your coins" is not just true technically, but true legally. If Coinbase were to become insolvent and decide to use your coins to repay their creditors, you might say, "Hey, you can't do that, that's my money!", and they would-- correctly-- respond, "No, it isn't. Read that user agreement again." They really are not your coins: Coinbase is extending you a temporary permission to access them which they can revoke at any time (and would, if they were in trouble). By contrast, as another commenter alluded to, fiat banking has an extensive system of laws and precedents that decides what happens in cases like this, which crypto lacks.