I know its not exactly the model for unbroken success, but Blackberry has been there, done that, with respect to sideloading. They enabled it long ago. It was not exactly drag-and-drop simple, but over time a few developers developed utilities to streamline the process. It was certainly an added value for people who were pushing to get more out of their devices. It certainly enhanced the beta-leak-testers' ability to continually upgrade their OS's, as it often became necessary to sideload different versions of apps when they would not function with an early release of a new OS build. The Blackberry community, best exemplified on Crackberry, of course, were tireless in seeking out the versions of apps that would work on new builds for the various devices. There was the threat of piracy, I suppose, but my sense is it never materialized as a real problem. More often, sideloading enabled people to acquire apps legitimately (through, for example, Google Play or Amazon) and then to finally be able to install them on their smartphones even when the developers couldn't be bothered to make a BB version of the app available. A lot of these apps tied into subscription services also, which was ultimately to the developers' benefit. On a few occasions, I talked the developers of an Android app to send me the apk and I was able to get it up and running.
Maybe the sideloading settings in the TP releases are a forerunner of MS's new exploration of running Android and IOS apps.