Don't those arguments apply to NCP too? i.e. You need 2 ports rather than 1, and there's probably other disadvantages, but that doesn't affect the average Internet user much.
Keeping IPv4 alive today doesn't hinder progress, and with the high bandwidth available nowadays the minor overhead improvements of IPv6 will be barely noticeable for the average user. Also keep in mind that the average internet user back in 1983 were a bit different from today ;-)
edit: typo
Because many users/services have only IPv4, practically all services and ISP have to provide IPv4 and do not have to provide IPv6. If we could agree to kill IPv4 at specific date, we could have truly IPv6 Internet years ago.
Anyway, it's not like it's a choice unless you want to restrict who deserves public IP and who doesn't, because we do not have enough IPv4 for everyone.