Conventional wisdom admires companies who have retained employees for many years. Trying to retain employees after they've stopped learning is bad for the employee and bad for the company. Bad for the company because it stifles the flow of new ideas, new insights, new views of the changing business environment. Your star employee, once a dynamo, once willing, able and eager to tackle any task, no longer shows that drive. However, it's more likely that he's simply no longer learning at the rate he was.
People enjoy best doing what they do best.
But most people don't get the chance to find out what it is they're good at. Say you hire someone in to do assembly. Forget any notion that there are men jobs and women jobs. And question any notions you may hold about skilled jobs. If your needs are routine, hire a moonlighting machinist or technician to come in to show your employees how to make those parts or how to run those tests.