You can't pretend like zero-confirmation transactions have never been useful. They have been used in practice for small amounts, many times, with no issues, and this system obviously changes things.
They are useful in customer-merchant contexts because the merchant can cancel the order after anyway in case of a double spend, and in in-person contexts because you're in person so the risk of trying to cheat is too high. That's the main reason why zero-confs are fine; you only really need the preliminary notification to be instant, not total security.
Yes, in financial transactions the standard is to use the credible approach instead of the expedient approach.
They have been used in practice for large amounts to double spend gambling sites that allowed them.
Wouldn't that present a horrible UX in some cases when paying with bitcoin? You'd have to wait ~10 minutes to access your purchase while the transaction is confirmed
No, there is credibility demonstrated by actual practice. Merchants accept zero-confirmation transaction, specifically in-person. Yes, there's a risk that the transaction never gets in the block or is double-spent, but that risk for certain kind of transactions is low enough to be tolerated.
I have bought cookies in Mexico with zero confirmations. So there was some credibility. At least with small transactions.
You're going to reverse a bitcoin transaction on some cookies you bought? That, sir, makes you a jerk.
What? No, of course not. I'm just saying that, at least, for some transactions, there is some credibility.
Not for small cookie transactions. Who wants to be "that guy?"