Just to clarify, I do not think that orangeoxidation holds the position that it is technically impossible. I just think that there is no evidence that it is either technically impossible or – like you – that there is any evidence or good reason to believe that sideloading would turn iOS into the wild west.
Rather, I think that the argument is sloppy, lazy, and frankly relies on a severe lack of imagination and rigidity of mind from those that present or accept it. Thus we should stop parroting it as it makes for a very boring discussion.
Instead, maybe there is an interesting discussion to be had as to whether Google maintains control using Google Play Services? Technical aspects of how a consent mechanisms could work relative to bootloader unlocking? The viability of Apple’s offerings in terms of better privacy and payment UI if something like Fortnite “broke free” and started a competing store? All this sounds endlessly more fun to read and contribute to compared to: “I am content with the status quo, it serves me well, and I see no reason to discuss this further with you freedom zealots!”.