> And when your phone dies???

You use the same procedure you used to register that phone, to register a new one.

> This rules out access from a desktop doesn't it?

Using the phone for 2FA means you are accessing from a desktop, and using the phone only as an extra confirmation for the transaction.

It also happpens when ordering stuff online depending on the level of security used by the online shop.

If your phone die, you replace it or do your operations at the ATM or at the counter during opening hours.