It could have been done with software updates only, to routers and hosts. The problem is, you can't just update IOS on your routers, you also have to add IPv6

It could have been done with software updates only, to routers and hosts. The problem is, you can't just update IOS on your routers, you also have to add IPv6 configuration to the routers. If two 32 bit addresses are talking to each other, use IPv4 packets. If at least one address is 128 bits, use IPv6 packets. It could have been that simple, with software updates (and hardware, in the case of many routers) over the past what, 15 years, we could have been running ipv6 effortlessly.