Who is to say an alternative App Store wouldn’t have better security than the Apple App Store? Lots of things with terrible privacy and security practices have made it past the mostly automated review process. With a 3rd party paid App Store, you could pay a subscription to ensure that every app is reviewed by a human, doesn’t violate privacy, and is free of malware.

Right, but the scammers aren't going to tell you to go there to install your Important Software Update, they're gonna point you towards the store with no restrictions at all, possibly one they run.