It's not illegal, and Apple wouldn't shut down the account.

Many companies do this with games that have a strong online component.

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"?

I'm not sure why Zune keeps coming up as an example in these threads. It got rebranded to Xbox Music, but all the servers are still up and anyone who subscribed still has all their music.

An earlier Microsoft DRM that was used by a bunch of services (PlaysForSure) was indeed shut down, but it's not the same thing as Zune.

Oh, I was thinking of the earlier one. I suppose it's often conflated with Zune, because of the spectacular flop of the Zune devices/rebranding thing.