The difference is that if I buy amd64 and decide to chuck 64-bit os on it, I won't have to touch my config files at all - they will just work from my 32-bit backups. Ditto most of my data. Also, my old apps will just work. And new ones too. Guess what? That's exactly what I did.
Right now, I have an OS that supports IPv6. It is useless to me without reconfiguration because I simply cannot participate in the IPv6. And why would I want to? This is a whole other network that has nothing on it. And that's because other people see the same.
In summary, amd64 is an inclusive technology. IPv6 is an island on its own.
You asked and answered a bunch of useless technical question, but you fail to see that the answer to a very simple and common question is still the main problem with IPv6. Why does everyone already connected to the net have to reconfigure and essentially run two setups? I can answer that for you if you like. Because someone wasn't thinking when they designed IPv6.