How often do you create new accounts at FIDO-supporting sites? You must have a different internet usage model than I do.

You can also add the additional keys later. If it's a serious enough account to be protecting with hardware keys, and you only have a couple keys with you, add a note in your password manager to remind you to add your other FIDO keys. You could add that note to the username so you can't login without editing the username and acknowledging it.

It sounds like a non-issue you're turning into a hypothetical concern because... I don't know, you don't like paying a quite modest amount of money for increased security?

Falling into some water won't brick a yubikey anyway.