The app market is by design extremely volatile. Basing your whole business model on apps (photo editing apps, in this case...) is risky as hell. When you look at it, most consistently successful applications are clients for popular websites (Facebook, Twitter), companion apps for services (Uber), or games. Those are things people won't abandon on a regular basis. Simple tools like photo editing will get an initial adoption wave, then will either disappear or maintain their userbase, both things that don't pull in any revenue - which is why so much apps switched to subscription models instead of one-time purchases.