The question isn't whether anyone could choose to stay inside if they want to, the question is whether I can trust that {insert older relative here} will stay inside the garden and not get tricked by a sketchy website into installing something through the back doors the EU is mandating.

If the opening up of Apple were as difficult to use as getting root on Android is I wouldn't have a problem. But that's not what's being proposed, and any attempts by Apple to make it less than perfectly smooth for someone to exit the walled garden are most likely going to be shot down.