I think it would depend on the jurisdiction, and the "degree" of "implied promise". Eg. in Norway there's pretty strong protection for (private) buyers. On the other hand, proving fraud in the case of the company behind the app simply declaring bankruptcy/shutting down would probably be pretty difficult.

I suppose a relevant study would be Microsoft Zune/music store, and/or Yahoo(?) Music that shut down and killed the DRM servers, effectively removing music people had "bought"?