Sorry, but if you do not believe getting security warnings, having to go into the settings and mangle with configuration, just to be able to install a store downloaded from a website, sometimes go through additional configuration depending on your phone's modifications (looking at you Xiaomi) to then install the actual program you wanted is not "discouraged by the system UX", despite the real world experiences of companies who have tried even simpler things and failed, I'm not sure we can agree on what's viable or not.