Question is, do any apps, other than a tiny percentage at the top, get a significant number of users that discover them via the app stores?

My guess would be most apps are discovered through other channels, and then the user just clicks a link on a website that takes them to the app store. Or they already know the exact app they're looking for, and search for it by name directly. So the store doesn't provide any value in such cases, yet they still take the 15% or 30%.