There is a legitimate concern that if the smarter programmer quits the job, the remaining average programmers will not be able to maintain the code.

I think a smart solution would be to teach the average programmers the new concepts. Many of them would probably be happy to learn, and the company would benefit from having everyone know a bit more and use better solutions. But for some reason, this usually doesn't happen.

>Many of them would probably be happy to learn

No. Most people don't like being told that they're wrong, drama ensues. They say they want to learn, but in reality, they don't.