That was basically what I was going to say too. The most secure keys are the ones that can't be transferred. If I want protection against the loss of a hardware

That was basically what I was going to say too. The most secure keys are the ones that can't be transferred. If I want protection against the loss of a hardware security key like a Yubikey, I have to get a second one and register that one as a secondary key to log in. Technically you can generate a GPG key and load that onto multiple Yubikeys, but that's not as secure as letting the Yubikey itself generate the key. Plus if you only use one GPG key and one of the Yubikeys is lost (but not destroyed) then you have to rewrite all the remaining Yubikeys with a new GPG key since you can't selectively revoke them.