> There is always an elaborate, sophisticated, technical explanation, usually many pages long. In the end, the simple question asked at the beginning remains unanswered: why do people connected already cannot just stay connected?
The short answer to "why do people connected already cannot just stay connected" is "because the IPv6 network is a different network".
Of course, this is not the question you meant. What you mean to ask is, if I am understanding the massive sprawling thread forest (but please correct me if I am wrong), is something like "why could not a transition plan be made which would allow for the IPv4->IPv6 transition with no loss of connectivity at any moment". With an implied "if everyone followed DJB's plan, it would have worked".
Leaving aside DJB for a moment, this is NOT a simple question. It is also VERY technical. Either you use a heavy amount of jargon, which can be impenetrable to anyone who does not have a deep knowledge of the Internet architecture, or you use a simpler (but still very technical) language and your answer becomes quite long. To make things worse, most of it would be showing hypotheses of possible plans or parts of plans and explaining where they fail.
As to DBJ's plan, here is why it would not work: "Once these software upgrades have been done on practically every Internet computer, we'll have reached the magic moment: people can start relying on public IPv6 addresses as replacements for public IPv4 addresses."
With DJB's plan, we could only START using IPv6 addresses after PRATICALLY EVERY INTERNET COMPUTER had been upgraded to understand IPv6!