>And why are you putting the blame on v6's designers, rather than putting it on v4's designers for failing to make their protocol forwards-compatible?
1. The v6 designers deliberately chose not to make their protocol compatible, thinking that migration would simply require the throwing of a great big switch. When they started this might have been possible, however by circa 1995 it was no longer a feasible option; yet the v6 designers persisted and continued to ignore the realworld.
2. It was only much later that more sensible people gave thought to interworking and migration.
3. The v4 designers did give thought to forwards-compatibility - it because of this compatibility that v6 traffic can be sent down the same pipe as v4 traffic and intermediate and end systems can pass the traffic to the relevant stack.
What is perhaps more relevant is given MS has supplied v6 since circa 2002, along with many (enterprise) network equipment vendors, why so few ISPs actually offer dual protocol services as standard.
Aside: I note that many consumer DSL routers etc. only support v4 and it is now 2018.