Aside from email accounts potentially being compromised or SMS interception, this is honestly the way all sites should be going now. But more seriously, there should be a way to use only Passkeys with a backup identification method in case you lose your Passkey.
The new threat? Browser password managers are insecure. Anyone can sit down at my machine if it's unlocked and Passkey their way into any of my accounts. What good is that? Why doesn't Chrome use my Google account password before allowing auto-fill/login? (Obviously I don't use Chrome's password manager, but it's a real concern for everyone else.)