... But that's no different from IPv4. Sometimes you have one per user, sometimes there are ~1000 users per IP.

Most of the ipv4 world is now behind CGNAT, one user per ip is simply a wrong assumption.

Anonymous rate limits for us are skewed towards preventing abusive behavior. Most users do not have a problem, even there is a CGNAT on IPv4.

For IPv6, if we block on /128 and a single machine gets /64, a malicious user has near infinite IPs. In the case of Linode and others that do /64 for a whole data center, it's easy to rate limit the whole thing.

Wrong assumption or not, it is an issue that is made worse by IPv6