There's nothing about <keygen> specifically that requires a central signing authority. You could just as well generate a public/private pair, feed the website your public key, and sign challenges with the privkey to log in.
1. Education for and purchase of U2F keys 2. Key loss recovery mechanism 3. Key stolen defense (you can't just rely on the U2F key alone, there must be a pw or other type of second factor) 4. Widespread browser & device support (without it, a user/pass is required as a backstop)
Nevertheless, it is progress.