> They would already be on IPv6, not caring one iota about the new connections having real IPv6 addresses.
You can repeat that 40 or 50 times, but it doesn't make it more true the more often it's repeated.
I will again claim that even with your version of the transition, ISPs still wouldn't have bothered to upgrade their routers to support the longer addresses faster than they have been doing now, nor would the manufacturers of dsl routers, cable modems, etc have upgraded their software (and configuration protocols) to support the long addresses any sooner.
They wouldn't bother supporting long addresses because nobody would use long addresses. Nobody would use long addresses because you couldn't talk to the majority of the internet (the part that still had at least one router on the path not supporting long addresses or with a host endpoint that didn't support long addressing). Until there was actually a forcing function, such as running out of short addresses. Just like with actual IPv6.
But, nobody can prove any "what ifs", so maybe it's time to just drop the whole discussion.