I don't think even many of the IPv6 designers would agree with you. Originally, IPv6 was supposed to bring a large set of improvements, like first-class encrypt

I don't think even many of the IPv6 designers would agree with you. Originally, IPv6 was supposed to bring a large set of improvements, like first-class encryption, larger minimum packet sizes, stateless addressing and auto-configured routing. It wasn't just more bits. In the end either all those things were backported to IPv4 or turned out to be bad ideas. IPv6 ended up being just more bits, but with a fully incompatible implementation. If they had known this at the beginning, we could have transitioned to IPv6 long ago, by designing for a smooth transition period and reusing as much of the IPv4 stack as possible. An end-to-end extension header would have been simple - something that can fallback to NAT connection tracking when it's not recognised. And it would have saved man-centuries of effort.