I think everyone agrees that not only Apple has the ability to offer an malware free App Store, it’s just that if F-Droid exists than so can G-Droid, the fork made by criminals to exploit less technical users. I think it’s overblown, though. Android has side loading and there doesn’t seem to be an epidemic of sketchy app stores.
Anyways, F-Droid is a clear example of how alternative sources of apps can be just as secure as the official one (and in F-Droid's case, more secure).