How much would it add to the pain of the v4-v6 transition, to just bite the bullet and do tcp-sctp at the same time? I'd sure rather be a network troubleshooter going through that than living with NAT forever.

it's the delta between the finite and the infinite. sctp requires a flag day whereas nat can be deployed incrementally. sctp isn't going to happen, at least not on the scale that nat has happened or that ipv6 will happen.