Let's start with one point. Third party stores don't have to be popular in order for them to be beneficial, because their existence causes the dominant store to take competitive steps to avoid their attaining popularity.

For example, you can install actual Firefox on Android, from Google Play, instead of it being nothing but a skin over the system browser like it is on iOS. Because if they didn't allow actual Firefox in Google Play, it would still be in F-Droid or similar, and that would make the competing store more popular. Since they don't want that, they allow it in the default store. Then the default store remains dominant, but only because it does beneficial things like that as a result of the competitive pressure. And third party apps can send and receive SMS on Android and various other things that Apple doesn't allow solely because it pushes people to use Apple's apps instead and there is no competitive pressure from other stores pushing Apple to allow those competing apps in their store.

But more than that, the other Android stores are actually useful, they just tend to have a niche. Because people are still going to use the default store unless there is a reason not to, since it's lower friction. But the other stores still get to address the cases that the default store fails at. So the fact that they exist at all is proof that they're doing something useful, since otherwise who would sustain them?

> I'm not sure how having to download app specific app stores (i.e. Epic's app store) is better for anyone except for Epic.

I mean, the fact that they're not paying Apple 30% and then passing most of that savings on to the customer is obviously going to save the customer money.