Even if NAT will be gone one day, the stateful firewalls won't. Every every home router would still ship with "deny all incoming" by default, and every corporate network would have the same setting as well.
Same as IPv4, IPv6 serving would still need registration with border device, either manual by user, or via UPnP-equivalent.
You're right, they are one level above.
> Hosting service A shouldn't mean that every user of service A can also figure out you host C, B and D.
It how are ports on a single IP address essentially different from multiple IP addresses within a subnet?