How do you convince good programmers to spend their life writing lisp? It's very hard to find lisp work after your company, so it kind of wasted years.
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.
For many journeymen developers, the choice of programming language is important. There are many, many companies out there that do look for people with N years of experience with language X and not every developer out there is in a place or at a skill level where they can afford to be picky (the average HN developer isn't representative of this population).
