A future OS update can break app compatibility if the app is not updated, apps not on the store won't be updated

Restoring a device from cloud backup will download all your apps, except apps no longer on the store

The device right now, with the OS right now, with the app right now will still be able to use this app and its file sharing feature in perpetuity which is an aspect that isn't different or worse than other kinds of devices or computers. Only the system restore embellishments being different or better or worse. People on other systems that pay for apps can still run into scenarios where their updated operating system is not compatible with an app they paid for. I'm not trying to be an apologist for things Apple or the mobile landscape could do better, only pointing out the standard people seem to be asking for without realizing it.

My understanding is that if the developer certificate is revoked, then the app would fail to run. But otherwise, apps on the device should run even if they’re removed from the App Store (by Apple or by the developer).