Yes, as long as you don’t upgrade your phone. At some point an OS upgrade will break the app.

I'd like to know this as well. My guess is that you may have to install it on your device before it gets dropped from the Apple store.

Definitely have a cloud backup of your device enabled in case you need to ever reset your device so that you don't lose the app.

If you want to install it on another device things become more complex...