Do you think Android will one day become so restrictive that sideloading (installing apps outside of Playstore) will become extremely limited, or completely banned?

Doesn't Apple already allow side loading to some degree?

You can just put an app onto a iDevice through iTunes, without having to run it through the App Store. Apple even puts out a specifically outdated version of iTunes that still retains much of the App functionality.

It's not as though they're trying to build the feature in from scratch.