Comment by Bluecobra

> We might have been able to migrate to IPv6 wholesale when the Internet was much smaller in the early 90s.

In the early 1990s IPng/IPv6 was not yet invented, and when it was being considered they realized a flag-day (like (mostly) happened with NCP->IP) was unlikely:

      We believe that it is not possible to have a "flag-day" form of
      transition in which all hosts and routers must change over at
      once. The size, complexity, and distributed administration of the
      Internet make such a cutover impossible.

      Rather, IPng will need to co-exist with IPv4 for some period of
      time.  There are a number of ways to achieve this co-existence
      such as requiring hosts to support two stacks, converting between
      protocols, or using backward compatible extensions to IPv4.  Each
      scheme has its strengths and weaknesses, which have to be weighed.

      Furthermore, we note that, in all probability, there will be IPv4
      hosts on the Internet effectively forever.  IPng must provide
      mechanisms to allow these hosts to communicate, even after IPng
      has become the dominant network layer protocol in the Internet.

That was the plan. V6 was killed by NAT and the commercial forces that promoted its use.

Not really.. IPv6 was theoretically ready in 1997. But, it was theoretical. It was still buggy. In 2000s Internet expansion skyrocketed and noone really cared about IPv6. Buggy, too different from IPv4, basically overengineered. They made prediction that was absolutly off.. And thats why adoption is crappy.