Same way. A majority generally have a old phone that was already signed in to google. Or if they remember only Apple ID and password (one very difficult password) + sms. They can login on to a new phone.

Everything is SYNCED immediately.

What if you have a ransomeware that destroy everything on the same day your house and all backups burn down. And you cant get it from immutable backups as you wrote that decryption key in paper. And the bank will not allow you to access it as the govt deported you elsewhere.

I can back up my FIDO2 (non-)resident keys too. In the end it's just a piece of HW with some secret material inside. non-resident FIDO2 keys are easier to back up, because the master secret seed is fixed and shared for all origins and there's nothing stored on the key.

I can get another hw key (if one of mine breaks, which I, surprisingly enough, have not yet managed to achieve)

And since I have at least two at all times, the possibility of one of them breaking changes… not much really.

Paper can burn or get tossed, backups files can go corrupt, and I really don't understand what is that extra risk hw keys introduce…

There is at least one valid (in my opinion) reason to not like hw keys though: they cost real money to acquire, so you probably want an extra margin in your budget for the unlikely case they indeed decide to break.