The same way I convince them to learn an employer's large code base that will be useless to them after they leave too: by paying them. That's a real, non-facet

The same way I convince them to learn an employer's large code base that will be useless to them after they leave too: by paying them. That's a real, non-facetious answer. Another facet to the answer is that many people see value in learning a different language to help give them perspective on the existing ones they already know. I generally don't consider it a net loss to ever learn a new language.