I'd argue that if the data is that specific to the app (e.g. which trees and courses of Duolingo you've completed), then if the app dies, the data is irrelevant. Either the data is useful outside the context of the app, in which case it tends to have standard formats (.png for images, .md for formatted text, etc), or the data is not useful outside of the context of the app (e.g. Duolingo) in which case the data is no longer of value and can be discarded, as no-one is going to be able to offer the exact same courses and trees and in-app cosmetics and gems etc etc etc.

It's true that the limited period of time between end of life and servers going offline may be a problem, but that tends to be a long period of time (3-6 months in general from the services I've used that have shutdown).