Amazon is the same as Facebook, they are a competitor.
My point was there is no exodus from the App Store. Sure, you can find a few companies fighting Apple but the fact remains the App Store earns more for developers than any other store, by a wide margin.
Competitors or not, they - evidently - support the platform. In that sense, they are no different than anyone else. They have content. Apple have a platform for said content. Simple, really. Competitor or not, no one wants to give Apple more money than necessary. Further, no one would want to pay Apple to do X if they think they can get X done cheaper through other means.
As for your claim, it is one you cannot support - not even in theory. Yes, the App Store is dominant in terms of quantity. But as "developers" are individual entities rather than a collective, these things matter little. In fact, some evidently earn more money on platforms other than iOS. (A phenomenon that is quite easy to explain using economic theory, as saturation leads to commodization in turn creating margin-pressure making development efforts for other alternatives ever more so interesting).
Last, and once more, i am talking about the beginning of something. Im not saying that every company on earth, or even most, have abandoned the platform or the App store. What i am saying is that were seeing a trend in relation to a particular solution (mainly, in app purchases - specifically, subscription services). This is correct. This can be theoretically supported drawing on e.g. economic theory. How it'll turn out? Only time can tell. Personally, i believe Apple would be better off not fighting technological evolution, embracing it instead.
Also. just to be 100% clear:
Yes. The App store has clear value. Both for users, and for developers. Further, the back-end part offers clear value for both users, and for developers. As result, there will be people happily paying Apple for these services. However, when it comes to content, these people are often quite irrelevant. Big content is owned by big media. Big media have money and balls. Big media can attain these services at a fraction of the cost, and are not as dependent on Apples services front-end.
Keeping ownership of platforms is easier said than done. IBM thought they had things in check, BIOS being their proprietary solution and a requisite for running DOS. How that one played out should be known to all. Safe to say, BIOS today is not "the platform". At times, Apple seem to show off the same arrogance and ignorance. In doing so, they risk bringing forth a future in which their own platform becomes of lesser value (e.g. apps as industry assets; web as the platform).