App stores, like payment processors, have economies of scale in the sense that you need a critical mass of users to sign up for them and trust them with their payment info, so you're not going to have a thousand of them, but you could certainly have a dozen. At which point they would have to compete on things like fees and keep each other in check.
But not if each platform has only a single dominant payment processor.
Unless I'm mistaken, Google has made threats, such as "if you include a competing app store, then you don't get any of the google apps, google play services, etc, and you can't use the Android trademark.
We saw in the early 00s a proliferation of "Android" devices that didn't have the Play store, and they were mostly DOA
An unwillingness to co-operate is not generally considered a threat.
> and they were mostly DOA
Maybe those stores should have invested a few dozen billion dollars in building up a comprehensive first-party app ecosystem that made them attractive to users.
Economies of scale, and all. Nobody's going to your farmer's market, because everyone's going to the mall that has a Walmart.
App store monopolies aren't built on threats. A mall giving an anchor retailer a sweetheart deal isn't threatening nearby malls.
They are, however, built on economies of scale.