There are so many business models and uses for 3p app stores: protocols, curation for kids, curation for niche tastes, a way to distribute news to benefit newspapers, new tech ideas, nonprofit certifications like low carbon footprint, etc!
This aligns with another thing I've been thinking about (but perhaps harder): in the EU, you can now have alternate app stores on your phone. What if we had on-protocol app stores? @trezy.codes covers a lot of the bits below, content moderation handles safety, social graph recommendations. Neat?