Even if that were true, all of your gain in wealth doesn't have to come out of a single individual's pocket. If you make something (e.g. an app) for which a lot of people pay a very small amount, presumably because they get some value in return, it seems a fair deal and leaves everyone else very marginally poorer, and not unfairly. If you consider the value they have gained from using your product as a form of wealth, then no one is poorer at all because you have created wealth.