Overkill is good! It's impossible to get the size exactly correct; your only options are "too small" or "too big". Given how difficult it is to deploy a new L3 protocol, why wouldn't we pick the size that ensures we don't have to turn around and do it again immediately afterwards?

> backwards-compatible would have gotten us there that much sooner

v6 is already backwards compatible. Between dual stack, Teredo, 6to4, 6rd, 6over4, ISATAP, 6in4/4in6, NAT64/DNS64, 464xlat, DS-lite, MAP-T/E, 4rd, LW4over6 and probably other things I'm forgetting, you could make a reasonable argument that it's too backwards compatible, even.

If you meant "v4 being forwards-compatible would have gotten us there sooner", then yes, I agree. It's unfortunate it's not. That's entirely v4's fault though, not v6's.