Doing what you set out to do and then moving on. How to do this in a way that is sustainably profitable and prevents someone else from coming along and eating y

Doing what you set out to do and then moving on. How to do this in a way that is sustainably profitable and prevents someone else from coming along and eating your lunch for as long as possible, is something I would very much like to learn how to do. Is there something wrong with just doing one thing well, making some money from that, and then moving on to do something else well and making some money from that too? I know it's an old-fashioned idea that something you do in tech might not continue to bring in rental income indefinitely...