That’s thing it wouldn’t work. It would have broken permissions and would never be able to properly function. Just like the location api changes etc. If try to launch an ios11 app like Instagram in ios14 it would be completely useless and nonfunctional because of just permission api changes. Not to mention any of the million other things Apple changes. So ya that’s this 24 hours is a crunch because even if 15% update the first day that’s 150+ Million Devices.