This is true but I think there's a key difference which is worth considering if you're in this situation: app stores are in the business of selling apps. This means there's a risk of being Sherlock-ed but in general they're not going to decide that they suddenly want to stop existing. They want you to sell lots of copies since they get a cut of every sale: if you're successful, they make a ton of money for a modest level of expense.
Something like a Twitter client is not only not part of their core business of selling ads but diametrically opposed to it so you have a much higher risk of unexpectedly being out of business because their success is not directly linked to yours. Arguments about innovation, bringing in super users, etc. are good but they're more tenuous and exactly the kind of thing some executive will dismiss when compared to the money they can make right now with more ad sales.
Neither of the major app stores is in the business of selling apps. Apple is in the business of selling iPhones and Google is in the business of selling ads.
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