You are missing a key point.. What is duolingo's success measured on? Not on how well their customers learn (they have already paid at this point).. but how much content is there (This number directly correlates to the signups). I am sure there are numbers around the subscribers vs regular content consumers and that ratio is tiny.
For a lot of these kind of apps, quantity is the quality in itself.
I suppose it sorta makes sense, if you think of it as a free mobile game model, where the goal is to make more money by get more people to pay (and especially by getting the whales to pay more) and to do that you need new daily or weekly content. Although, I feel like that model scales just fine with humans creating the content.
For a lot of these kind of apps, quantity is the quality in itself.