Case 1 and 3 would be new business, you'd still have to convince those users that Instapaper is right for them, there's nothing automatic about it. Also what makes you think Instapaper is the popular brand on Android?
Case 2 I'll concede but that's hardly "a lot" of users.
- users that have switched from an iPhone recently and have been using that
- users that have seen the app on a friend's phone
etc ...
Seriously now, if you built a game now, and got EA Games to publish it for you, wouldn't you like to get 50% of the profits? Not anyone can build a minecraft