What if those apps moved to other stores so they can skirt Apple's review and other consumer-friendly restrictions? How is that better for consumers that use Facebook, Insta, etc... for them to have apps with less review and less scrutinized for their behavior? Some of Apples policies have been good for consumers of apps.
Just witness how Fb, etc... already try and skirt those rules that are in place to protect users from tracking and other abuses. Seems pretty logical to assume they would all jump ship to another store to not be under Apple's review process if they could.
I don't doubt for one minute that Fb, etc.. would not jump to another store with less restrictions, and either pull their existing apps or leave them severely restricted in the Apple App Store as an "incentive" to download from the other store.