> Put their app on the niche store and not on the main store (which would lose them money; fewer people will go to a niche store and buy a paid app than a freemium app. Therefore, developer have no incentive to do this)
You're assuming that all customers want the same thing.
Right now if you want to make a paid non-freemium game, no one can find it because the freemium games will have more downloads and the main store sorts by popularity. The customers who want such games hate this too, but because the main store has no competitors, they're just as stuck with it. And since customers can't find them, developers don't make them.
If you had a store that has only non-freemium games, the developer who makes one can go there and actually reach their customers, because being at the top of the list on a store with 1% of the market generates more sales than being on page 5000 of the main store's search results where even fewer people will see it. And then more of those games actually get made, which grows the market for the alternate store, which allows more of those games to get made.
Meanwhile that store doesn't take a huge chunk out of the main store's market, but it takes a percent or three. Many other niche stores each take a little share. And then a big store that competes directly with the main store but only charges 3% instead of 30% takes the rest.