I suppose that you could argue that packet switching won out over circuit switching. I mean, except for the gigantic installed base of landline telephones (anal

I suppose that you could argue that packet switching won out over circuit switching. I mean, except for the gigantic installed base of landline telephones (analog and ISDN) and the digital switches that support them on copper pairs, still extant and supported, VoIP is incrementally replacing those lines as the decades progress. (How many decades has it been now?) But you're comparing apples and oranges. What was the size of the established, installed base of network hosts using circuit-switched networks when packet switching started to compete? Did SRI International and other DARPA research institutions run IMPs that utilized circuit-switching to route network traffic? No, they started with packet switching. So essentially, as the Internet caught on, its style of packet switching "won" over landline telephones because it was a question of capability and supported features. IPv4 and IPv6 are the same protocol, essentially; you swap one out under the hood, and the upper and lower OSI layers don't even notice that anything's changed. Packet switching and circuit switching are two distinct techniques that are opposed to one another: if you wanted to swap one for the other, you'd be rebuilding your network from the ground up. So, apples and oranges: IPv6 is, by design, a replacement for IPv4, and djb is discussing the seemingly intractable issues faced by those who attempt to cleanly migrate.