Owen DeLong wrote privately to me, but as I think I need public responses, I'm Ccing to nanog fairly quoting part of his response:
Moreover, it is easy to have a transport protocol with 32bit or 48bit port numbers with the end to end fashion only by modifying end part of the Internet.
The center part of the internet is the easiest part of modification for IPv6 and is probably somewhere near 99% complete at this point.
That is a fairy tale once believed by so many infants who thought dual stack were enough. They still believed the fairy tale even when they designed automated tunneling. But, as most of them have grown up a little not to believe fairly tales, they are trying other possibilities. However, so far, they are not so successful. Masataka Ohta PS Rest of his response is omitted, because I think it is not worth quoting.