Earlier quoted context omitted.
iOS devices don't support downloading specific versions of apps, and you cannot sideload. Apple has made this model impossible on iOS.
That technical method isn't the only way to implement this business method, and a number of existing apps are doing this quite successfully already. So this both already exists, and works. As for 'side loading', it's somewhat possible, but even if it was available by default it would be messy for most customers -- they already have trouble with what's currently available. Perhaps some sort of in-app store would make…
Based on the wording comment it sounds like there may be additional ways I haven't seen but you didn't really say what they were as much as dismiss the point assuming everyone knew what apps or release models you are familiar with.
I agree an in App Store solution like the one you describe would be the preferred answer for most users. Sideloading is more useful as a catch-all for anything Apple doesn't have a good first party answer for. Once non policy related use cases for the sideloading exist it's going to be easier for most to have a first party solution for those use cases.