[kbolino]

You would need to have a completely trusted software and hardware stack to actually own the keys. And that is already hard enough to get on a PC where ownership still means something, it is not going to happen on most mobile devices. To whatever extent you trust any of the stack already, the Secure Enclave is a better bet than BYOK. The real risk, as you imply, is if Apple is able to compromise the security coprocessor with an OTA firmware update, but they can definitely already push a regular OS update that exfiltrates any key you type in.

[codedokode]

Just make an airgapped Linux device on a DYI FPGA CPU. This part is not that difficult comparing to persuading commercial vendors let you use your own cloud and your own encryption/backup mechanisms.

[rkagerer]

Yeah... unfortunately it ought to be the other way around. They should have a hard time pursuading us to trust them enough to use theirs.

If your phone company asked you to give them the key to your house, in perpetuity, how would you feel about that? (Particularly if they insisted you sign a 15 page Terms of Use first that disclaims all their liability if anything goes missing).