At least in Android it is already the case, I have installed both F-Droid and Huawei's AppGallery in an otherwise stock android. (And prefer to get as much as possible from F-Droid)
Apple on the other hand has a war against "evil" sideloading. :P
Apple on the other hand has a war against "evil" sideloading. :P
There are roadblocks and deficiencies though, a major one being that sideloaded app stores cannot automatically update their applications, making them permanently inferior and crippled compared to Google's offering.
I don't know what the reason for this is, but the result is that Android is not a truly "open" platform.