I do wonder if things would have been fine if had just added another octet. We would still have had enough addresses to not need more for the forseeable future. Some may disagree, but a trillion addresses are fine for human population which is going plateau soon. I am not a network engineer, but I believe it would have been far easier to make the two versions interoperable. The number of addresses possible with IPv6 are too much for its own good. The ranges are absurdly long and far apart. Add to the difficulty to communicate and understand an address filled with hex-codes. I think IPv6 solved more problems than that were actually needed.