That’s a great theory but in practice such a “flag day” almost never happens. The last time the internet went through such a change was January 1, 1983, when the ARPANET switched from NCP to the newly designed TCP/IP. People want to do something similar on February 1, 2030, to remove IPv4 and switch totally to IPv6, but I give it a 50/50 chance of success, and IPv6 is already about 30 years old. See https://ipv4flagday.net/
Well, this is basically what we do, except that we try to negotiate to the highest version during the period before the flag day. This is far more practical for three reasons:
1. You actually get benefit during the transition period because you get to use the new version.
2. You get to test the new version at scale, which often reveals issues, as it did with TLS 1.3. It also makes it much easier to measure deployment because you can see what is actually negotiated.
3. Generally, implementations are very risk averse and so aren't willing to disable older versions until there is basically universal deployment, so it takes the pressure off of this decision.
You don't have to have everyone switch over on the same day as with your example. Once it is decreed that implementations are widespread enough, then everyone can switch over to the introduced thing gradually. The "flag day" is when it is decreed that implementations no longer have to support some previously widely used method. Support for that method would then gradually disappear unless there was some associated cryptographic emergency that could not be dealt with without changing the standard.