Would it have been feasible to devise a protocol which accepted IPv4 as a fully accepted subset? I don't know, but if it would then anything else would have been a serious mistake.
No, it wouldn't. There's no way to do what you're imagining. That's why we didn't do it.
Note that it is possible to connect from v6 to v4 via NAT64, and there's a standard range for that (64:ff9b::0:0/96). For example, in the process of making this post, my browser is talking to 64:ff9b::104.18.227.129, since for the fun of it I removed v4 from my desktop and am relying purely on v6.
Is that close enough to what you wanted?
It's impossible to do a completely seamless transition, because v4 is just not designed in a way that supports that, and obviously you can't change v4 to support a seamless transition because that wouldn't be seamless. v6's design makes the transition as seamless as possible given the constraints it's working under.
Biting the hand that feeds IT
