How to restore app unavailable to download from purchased list?

For example a video editor. Imagine after some years you need to reopen a project file but you have a new device, and the app isn’t anymore on store. How should I manage that?

From backups the app just never transfers since even iTunes backups now don’t contain the app itself, just the data. It just says “unable to download application” and you’re stuck with that.

Since it already happened with some apps, it really becomes hard to implement on a professional workflow where this kind of gamble is not worth, making any app from the AppStore a bit unsafe if they get removed, you also lose access to your previous projects after restoring a device (which is required due to some iOS bugs sometimes).