This statement would be more believable if Apple simply showed us how bad it would be running it on the A12X/Z (eg super choppy animation, apps crashing due to low RAM, etc). Simply demo it and say “see, this is why we require the M1.” Done. People would be more than willing to accept and even upgrade. Be transparent.
But no, Apple won’t do that, because they probably know it can be worked out if they wanted to. Put a limit on how many apps retained, and/or cut down some of the eye candy, or just have it not support external monitor in the older iPads.
It would be hilarious if somebody managed to jailbreak the older iPads and then enable the feature, and then it worked fine.