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Why sideload when you can simply open it up in any mobile browser?
Easy solution for Apple then: stop selling things labelled as "pro". I was already burned when I bought the 16 inch Macbook Pro, which was a shockingly incapable and undocumented device. Lenovo provides better documentation on their entry level laptops than Apple provides for their Pros, despite the order of magnitude difference in price between both devices. So to the "Apple Engineer" out there, I'd encourage you give people the choice anyways.
In other words, North Korea doesn't justify their atrocities by telling the rest of the world that their culture "isn't for them". Apple is no less accountable for their own product line, particularly when they're some of the most profitable electronic hardware being manufactured right now.
As for the term "pro" - nobody agrees what "pro" means. An art designer might do professional work on a MacBook Air, even though you wouldn't call that a "pro" machine. Although you might not like the terminology, you should have been able to research and know what you were getting, and it still absolutely does not bear the comparison to North Korea. First world problems compared to third world tyranny.
Easy solution for you: Accept that its not for you, even if you think you're "pro". Its just a word for marketing. You're the one with a problem in search of solution, apple is not.
If people are still using Facebook, today, knowing all that has happened, the odds are most people will continue using it though one can dream. I would honestly expect 90%+ of Facebook users to sideload if Facebook required it.
"But what about other areas? I don't want one single company dictating what I can or can't use on my device."
If you told this to an Apple Engineer, he'd look at you in the eye and tell you that the iPhone isn't for you and to buy an Android. You have that choice. The iPhone doesn't fit your use case and Apple isn't interested in making it fit your use case.