I may be bucking the 'everything will be mobile first/mobile only' mindset, but I see the 'successful' apps as those that fit in a larger ecosystem of functionality. For many users, the primary/only point of contact with a system may be the mobile app, but there will be other endpoints of the system with more traditional web interfaces or even other endpoints which will provide other value to other parties.
Yes, the instagrams and yo and such are 'successful', but ultimately, they tend to prove the exception to the rule, I think. Outside of gaming, I'd expect most apps to have other functionality that can be processed and accessed with in other ways beyond solely a mobile interface.
To that end, the mobile app experience doesn't necessarily have to be 'high-caliber' over and above all the other competition, as long as the rest of the supporting functionality provides enough value.
I may entirely wrong though...