2: If IPv6 were backwards-compatible, we wouldn't. We could go from IPv4 to IPv6 just like going from CDs to DVDs to BluRay. But it isn't and therefore we won't ever replace that structure.
IPv6 is backwards-compatible in exactly the same way that BluRay is backwards-compatible with DVD.
Your BluRay player has a BluRay VM and Java VM, and uses H.264 encoded video. None of that is part of DVD playback. There's a totally separate stack of code that handles DVD menus, MPEG-2 video, and interleaved MPEG transport streams. Your separate DVD software stack and BluRay software stack sit on top of a single piece of hardware for reading data from the discs. The UI then makes the distinction largely invisible.
And similarly, my computer has an IPv4 stack and an IPv6 stack, and they both sit on the same network hardware that reads the packets. And the OS makes the distinction largely invisible to the end user.