You need app Foo on your phone for work (like Slack, maybe). Foo is in the App Store and worked great on your iPhone but decides they want to install via their own store so they can monetize employees’ data (location, whatever) to make more money. The new store launches, the new app version abuses private APIs, and the App Store version stops working. Your company announces that all employees need to download from the new source. Do you really have a choice about staying in the walled garden? Sure, neither your company nor Foo should suck, but we all know plenty of companies that don’t care about employees or users.

Game company Bar decides to launch their own store and pull their game - we’ll call it Nortfite - from the App Store so they can add something shady like crypto features. Nortfite is a massive social game that all your friends play and it’s a huge part of your teenage social life. Your only device capable of playing it is your second-hand iPad. Do you really have a choice about staying in the walled garden? Who needs friends anyway, amirite?

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.