one can confidently say that even had NCP _not_
been turned off, history would have proceeded much as it actually did, since
all the machines not on the ARPANET would have wanted to be connected to the
Internet.
I agree - this is classic Metcalfe's law. Because no new NCP sites were being added, the Internet quickly became the more and more valuable. Which is exactly why IPv6 never flipped. We succeeded in keeping the old being more valuable than the new, so there was not real push.
I had hoped the backbone providers would offer a rate differential (i.e. make it cheaper) to use IPv6 because it should have been easier for them. I practice is not and none of them ever did to my knowledge. So the economics just there like it was between NCP and IPv4.