tcp6 is a thing though, was created at the same time as ipv6, and it does have ports, along with udp6. But if you really want one ip per stream and just hardwire port 1 or something, it's not like IPv6 does anything to stand in the way of that. Mght have performance issues on some OS's binding thousands of IPs to one interface, but that's on them to fix. Bigger lift would be the APIs that would need to change to manage whole prefixes at a time instead of single IPs.