v6 has dual stack, Teredo, 6to4, 6rd, 6over4, ISATAP, 6in4/4in6, NAT64/DNS64, 464xlat, DS-lite, MAP-T/E, 4rd, LW4over6... it has pretty much every possible backwards compatibility method that can work with v4.

You could make a reasonable argument that it has too many of them, even. Where did you get the idea that it didn't have backwards compatibility?

It doesn't have any backwards compatibility. You can't turn off IPv4 and just have IPv6 and still use IPv4 addresses. If it had it, you wouldn't need all those hacks.

Yes you can. Those "hacks" are how you do it.

I mean, you can call them hacks, but at the end of the day v4 uses a fixed-width 32 bit address field and has no mechanism to extend it in a way that's compatible with other v4 hosts. All you can do is hack around that. There's nothing that v6 could possibly do to avoid it, because the flaw is in the design of v4 and not in the design of v6.