[Dagger2]

Yes, of course they were all parallel protocols -- because your problem here is that v4 doesn't _have_ variable-length addresses. It's trivial to imagine a version of v4 that does, but that version would also be a parallel protocol to the version of v4 we actually have.

> even the idea of starting a new protocol with 64-bit addresses with an upgrade path was considered far too scary at the time

No it wasn't? Every proposal had an upgrade path. Having one was a mandatory requirement.

You can read the requirements document yourself: https://datatracker.ietf.org/doc/html/rfc1726. To me, it looks like these requirements were decided by the community rather than being imposed from above, but either way you can see that having a simple transition from v4 is listed right there.

> A decade later, when IPv6 had real-world deployments was far to late for 6to4 to save the day: entirely because a swath of non-6to4 addresses existed and needed to be reachable

What I'm hearing is that the compatibility with v4 that 6to4 provides wasn't considered important, and not by people in any position of authority but rather by the actual people choosing what to deploy on their own networks. Even though there were more 6to4 hosts than non-6to4 ones, and even though 6to4 doesn't prevent you from reaching those non-6to4 hosts, people still didn't want it.