Look a little more into the subject, and you will see that 6to4 does not offer backward compatibility in any way, shape or form. It is simply a way for IPv6 "islands" to communicate over an IPv4 network.
The real problem, however, is getting an IPv6-only host to communicate with an IPv4-only host. Until we have that, there is no backward compatibility to speak of. And so far, the only mechanism that can do that (at least partly) is a form of NAT - NAT64. And that is still in its infancy, works only one way (v6 client to v4 server), and brings with it all the disadvantages of NAT (which we are trying so hard to avoid by moving to IPv6).
Even though I do not agree with his solution to the problem (I don't think his multicast solution would have solved the problem), the OP (Alomex) is right - there is no backward compatibility built into IPv6. And this is what has hindered IPv6 adoption from the very beginning.