Yes, I think it would be great to be able to create the key otuside of the physical hardware token, just like you can do with the GPG applet on the Yubkey. Store the original on some offline, encrypted media, ideally in multiple places.

This way, you don't need to have two yubikeys on hand and register them both on every new site. But in case you lose, it's not impossible to create a new one. Yet, obtaining the key from the hardware token shouldn't be any more possible than with the current system.

This is what I would require before migrating. Opensource local only Fido key generation.

But like I said in the comment below, I suspect these vendors are only here for vendor lock-in and targeted advertising. And the websites enabling webauthn are likely to require you to use a vendors app/service. Like banking sites today not allowing me to utilise keepassxc totp functionality trying to force me to download their app or SMS.

Software-based passkeys can do this! Bulwark Passkey (https://bulwark.id) is a virtual Yubikey-like device, and you can back up the credentials file however you like.