And I'm fine with Apple siphoning off 30%. Make it 40%! I'd rather developers stop developing for iOS than to see any large changes in the App Store model. There are enough apps as it is.
While I don’t know the exact reason, this isn’t what’s happened on Android, which has always allowed multiple app stores. Everything mainstream is on Google’s Play Store, and quality FOSS apps get published to F-Droid. My theory is that users shy away from sideloading alternative app stores, even if it’s not very difficult to do so.
I don't think users would shy away from mainstream apps like Facebook if they moved to another app store platform.
The only thing a 3rd party store support will do is to ensure Apple couldn't abuse FOSS apps too much in their review process.