alt Hacker NewsIt was not possible to make a "superset" of IPv4, if only because one of the early major blockers was that BSD Sockets suck by leaking low-level details of addressing so you'd have exactly the same argument of "why should I bother writing entire second copy of connection code in my application" for any superset you want to imagine.
Similarly, we have 30 years of experience that vendors will happily break optional headers or flags.
I don't think this is how it would have played out at all.
I'm no expert on IPv4 or IPv6, but if they had designed IPv6 to be able to route fine to IPv4, we'd be OK.
It would at least give people an upgrade path where their old stuff that couldn't be patched / updated and were stuck on IPv4 could be slowly killed off in the path of least resistance down the dependency line.
This 'dual stack' approach doubled up on everything up front and meant we all had to do both during the transition (which has taken 30 years).