That’s why you should get two.
And if you only need FIDO2/passkeys, the Security Key series is half the cost ($25) of the Yubikey 5 ($50) and all you really lose is OpenPGP and PIV (smart card) functionality.
Now I like playing with all the features of the 5, but most people should just need FIDO2.
I looked into this a year ago and most sites did not offer to register a second key, so if you lose your key, you can kiss many of your accesses goodbye. I would never have the key to my digital life on a keychain… The idea is good, but it will cause huge damage if you lose your HW key. On the other hand, if you are cautious and use different PWs and a password manager with 2FA, you are quite safe.