Yes, I'm aware of this problem. First, we're unlikely yo be acqui-hired, because we're based in Russia. Second, I simply don't believe that this app will reach the levels of success similar to Wunderlist or Todoist :)))

But I definitely think that there should be a legal way of protecting existing users of an app from "Our Incredible Journey". Maybe some clause that permits the authors to open-source the app.

Are there any examples of this in the industry?

Isn't that what happened with Astrid? It was at least part open source and they released that code when yahoo acquired it.

Of course, the buyer may just ask you to remove that clause prior the acquisition.