People just add flags in their mining protocol that only trigger when a threshold is reached. The last Segwit changes needed a percent change of closer to 90% just to trigger the next change.
We are only assuming that consensus would be reached quickly given the scenario presented. It would be irresponsible to design it to need simultaneous action. People would have to considering to stop using the bitcoin network for X,000 blocks while consensus is being reached, and only until it is reached.
I don’t imagine this is other settings... “I only do breaking changes on my api so let’s have everyone change their client at midnight”.
Imagine if we all changed implementations of SQL at once!
Actually, we changed the meaning of the $() function inside the devtools console across all browsers at the same time. That was fun. :)