I'd be tempted to create the phone app but have any proceeds go to charity - that way you take the opportunity, gain kudos, avoid greed, and give something positive to the world. Perhaps also including a prominent link to Threes as an acknowledgement of their founding status and to help drive the popularity of their original app. That said, if to do that you need to put your regular income stream on hold to focus on the project, I can understand not going that route.