It seems to me that it can't both be true that (a) the appeal of the walled garden is wide and (b) sideloading or other uncurated channels are an irresistible f

It seems to me that it can't both be true that (a) the appeal of the walled garden is wide and (b) sideloading or other uncurated channels are an irresistible force that will inevitably obliterate the walled garden. If people find value in it, they will not migrate to uncurated channels, so there will remain a population that's only reachable via the garden, so developers will have an incentive to continue to sell there. And as far as I can tell, this is what's borne out by observable results in places where there are options. macOS has preferences which let you choose between (a) app store only and (b) app store + signed apps from other channels -- and lets you override both choices if you really really want (as well as having a unix command line from which you can run arbitrary things and the ability to acquire dev tools by which you can build and run untrusted source). Some people take safety into their own hands and use those things. Some people don't. This hasn't eliminated the Mac App Store as an option for people who want to rely on Apple's curation. It exists side-by-side with independent but still signed/trusted distribution, and both exist side-by-side with free-for-all. You can walk freely between the well-patrolled walls of the fortified city, the ring of civilization just outside it, and the wild west outside that. None of them stops the other from existing. Are there any platforms where that's happened? If macOS isn't a place where that's happened, why would it happen to iOS?