>So the platforms have to make it nearly impossible to keep everything in their controlled channel.

I don't understand what you're saying. Are you saying Google is making it harder to develop an app for sideloading than to develop an app for the Play Store? I don't see how that's the case. AFAICT, the new "sideloading" requirements aren't more restrictive than the Play Store requirements.

Disclosure: I work at Google, but not on Android.