It’s not just “sideloading”, it’s Apple being forced to break its vertical integration with the App Store. The point is that the App Store cannot even be a “preferred” source for apps, much less the only one, and that includes having a perfectly workable iPhone with the App Store not even installed ever, because you did not choose it from some list at first start, and you’re rolling with some third party store exclusively.
In theory, it should look like Windows and its browsers, or even more free, because what Windows does now with Edge is also illegal over the DMA and Europeans will soon be able to uninstall or not even have Edge installed on their Windows devices.