So far no one ever managed to come up with a plan that is backward compatible and does enough. (Aka the fundamental problem is that if you change v4 semantics you have to deploy new software/firmware/hardware, and if you are doing that anyway, a clean slate seemed like the easier option.) And don't forget v6 has well defined semantics with regards to v4 interoperability for a lot of things (::ffff:, NAT64, DNS64), so you can be on v6 and reach all of v4 via a gateway.)