In the more expensive devices, are the arbitrary new private keys imported from the computer it's plugged into? If the new keys are generated on the hardware yo

In the more expensive devices, are the arbitrary new private keys imported from the computer it's plugged into? If the new keys are generated on the hardware you don't trust, it'd still be the same problem, since the private keys could be generated deterministically from a known seed and a counter. You can at least verify when a cheaply-designed device has changed its secret key, because the public key it offers for github.com is different from before, but yeah, that 'new' secret key could still just be derived from a manufacturer-known seed/secret serial number, too, same as the first one was, but with an incremented counter.