Well, a more pragmatic reason is that over time, CGNAT will force more users behind shared IPv4 addresses, so websites with IPv6 will be able to do abuse detection more accurately.
Migrating popular traffic from CGNAT to IPv6 can also improve performance for some users, and reduce the long-term costs of CGNAT for ISPs.
I don't understand. There is no CGNAT in IPv6. You can do NPT (network prefix translation) if you really wanted, but why would you let users share the same IPv6 address/prefix as it is too much work (and not needed in any realistic setup).
Migrating popular traffic from CGNAT to IPv6 can also improve performance for some users, and reduce the long-term costs of CGNAT for ISPs.