How does Keybase address this problem completely? If an attacker gains possession of a key, they don’t have to change ownership, they just assume the role of that user. NB: I know they could feasibly only do this once before the key is considered burned by detection from the original user, so I’m somewhat lost here. Presumably you could mean that revocation of a known stolen key would be easy to point to, but any PKI can handle this.
Edit: I’m aware of Keybase itself doing per-device salts, but under a sophisticated attack, I don’t see a workaround here unless they’re doing some multi-party signing process with authorization against the Keybase server who acts as a second party, but even then, a sophisticated attacker who has control of the user’s machine impersonating someone would still be able to circumvent this.