Absolutely, yes, but if you are writing your own apps rather than being paid to write apps for other people, there's a strong luck element as with any entrepreneurial activity. I have six apps out right now, of which just one accounts for 90% of my revenue. Rather than setting out to write apps, I think it's better to think about an app as merely a vehicle to deliver a service, a product, some unit of customer value, from you to your customers. If you have something to offer that is of high value, you can definitely make a lot of money from an app. If you don't, no matter how well designed and built the app is, it is likely to fail.