It was engineered like that. It was known from the start that having a flag day wasn't possible (https://datatracker.ietf.org/doc/html/rfc1726#section-5.5 https

It was engineered like that. It was known from the start that having a flag day wasn't possible (https://datatracker.ietf.org/doc/html/rfc1726#section-5.5 https://datatracker.ietf.org/doc/html/rfc1726#section-5.5): 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. And when they picked a design, it indeed didn't have a flag day (https://datatracker.ietf.org/doc/html/draft-hinden-ipng-overview-00#section-1 https://datatracker.ietf.org/doc/html/draft-hinden-ipng-over...): IPng is a new version of IP which is designed to be an evolutionary step from IPv4. It is a natural increment to IPv4. It can be installed as a normal software upgrade in internet devices and is interoperable with the current IPv4. Its deployment strategy was designed to not have any "flag" days. If saying "we can't have/didn't do a flag day" in the design documents, and then not having a flag day, isn't enough to stop you from arguing that v6 should have been engineered without a flag day, I have to wonder what v6 could possibly have done to make you happy with it.