Unlocked boot loader would be fine. Sideloading's fine as long as the UI's bad enough that normal people can't, say, click a link on a website and keep clicking "OK" until the app's installed on their phone, and it also can't (somehow? Not sure how this restriction would be enforced) be scripted into a desktop-based store (as ADB could be, and likely would have been if alternative stores couldn't just be installed directly on Android devices).

Essentially as long as nothing threatens Apple's ability to tell other companies, including other tech giants, to play ball or pound sand, I'm fine with it.

As soon as Apple offers that, companies like Facebook will tell their users that in order to install their apps, they have to sideload it.

And people will go willingly into that dark night.

And there’s a reason for that. You’re making the mistake of thinking Apple’s special sauce is the hardware. It’s not.

It’s the experience.