Sorry, but you completely misinterpreted what DJB wrote. He was talking about upgrading everything on the net with software that understands both types of addresses. This includes routers. This was 8 years ago - plenty of time.
The reason this wasn't done is that IPv6 in its form (as proposed) was useless and not interoperable with IPv4, ergo nobody wanted to spend time configuring something that had no application.
The whole thing should have happened transparently, so that current IPv4 site didn't have to change a single thing to work with IPv6 addresses. If network manufacturers received that message, there would be no question which equipment to buy. It would be one and the same. And you would not have IPv6/v4 stack combos on OSes - just IPv6 that included IPv4. That's the point that you missed.