The biggest security benefit of passkeys is that it raises the floor of security posture for the least security conscious users. E.g. You can't use "bob1989!" as the your password on every single site, write it on a post-it stuck to your monitor, and enter it into every site that sends you an email claiming to be from "M1cro3oft Supp0rt".

For users who already have a very good security posture the security benefits are pretty marginal. For example, I already use a long, unique password for each site via my password manager. If a site gets breached it will only affect that specific site, and even then only if they store passwords in plaintext. My passwords have enough entropy that even an MD5 hash is unlikely to be cracked in an offline attack.