> But it's also been apparent that relying on code auditing wasn't going to be sustainable long term.

Why not? It seemed to be going fine. As the number of apps increases, so does get money the App Store receives. I don't see anything inherently unsustainable in it at all.

For the same reason that all central planning ends up being unwieldy - the world is complex. It's not the work of doing the reviewing itself that is unsustainable, but rather crafting some singular policy that can reject all "bad" while allowing all "good", compounded by having to work with governments/corporations/etc that want increasingly fine-grained censorship.