Earlier quoted context omitted.
> Can't speak for the Google store, but as a consumer, I definitely trust Apple's apps more than random developers' apps. Couldn't you still have that without banning off-store apps?
By default, you can't side-load applications on any android phone I've used in the last 5 years. It's not hard to turn it off, but it's also not obvious how and a consumer would have to be taken step-by-step on how to do it, and so the default Google experience is the same as Apple, sans the critical difference of much much more stringent app review on iOS.
As an end user, it's akin to typing my root password to clicking "accept" on Windows UAC. I can still install it just fine, but I need to give permission and check the box accepting responsibility.
Whether that's enough to protect someone from doing something unwise can be debated, but it's not something you'd really need instructions for.