As stated by the spec authors on KeePassXC's bug tracker, open source software may be modified by the user, so cannot be trusted. The passkey proposal is for all of your keys to be managed by proprietary software validated by your phone or your computer's TPM module. That means one of three big, US-based tech companies will control all of the world's login data. Those 3 companies are all currently involved in the largest fascist-taint-tongue-polishing in US history, and we want to hand them control over the world's logins. That's a much, much bigger risk than some users doing something stupid.

The spec needs to be written with the assumption that the user's private keystore may be hostile to the user's own interests, because in the real world, it is. It needs to be written to mitigate damage to the user from a hostile keystore. Instead, the spec places total trust in the keystore. This is a fatal error.