The guy making the mail client is not being retained as a full-time employee for his expertise by the people buying his mail client. It's not at all a similar relationship.

But to answer your question - I work on a variety of internal projects for a company that moves freight. Every year I have to justify my salary by either being budgeted to continue supporting a product or by moving to a new product undergoing active development. I'm hardly writing one product and then receiving a unending revenue stream from it.