They did give it thought. We have 6to4, Teredo, ISATAP, NAT64, 464XLAT, 6rd and DS-lite. We have proxies. We have a protocol that coexists cleanly with v4 on the same networks and the same hosts, with the same socket and DNS APIs and the same software.
What we don't have is "unmodified v4 host talks to v6 host"... because that's impossible.
What more are you asking for? What more could we possibly do? 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?