In my experience, when someones applicable experience is more than what is stipulated by the job requirements, they come up to speed much faster. I hired a brilliant engineer who did the new hire on-boarding exercises we reserved a couple of weeks for in about 6 hours.

I say applicable because having a PhD in Chemistry is not being "over qualified" for entry level programming. But having 10 writing code could be interpreted as having more qualification than the job merits.

Even if it's somewhat applicable (like coding in a different language), it's still much quicker than onboarding a college grad. Someone who knows C++ and C# can probably learn Java in a week or two, to the level where he can contribute at least as well as your moderately skilled employees.

These is also the appeal of developing in a well popular langguage/framework like .net or java. The hordes of experience devs available. Sure, you may not want to use these heavy weights when your discovering your product/market, but when you've got a fit and need to hire talent, it's nothing for a 15 year java dev to come in and be productive on day one using Spring, maven, intellij, etc.

> ...if it takes 6 months to get a new developer up and running on the codebase, and then they leave just when they are getting productive.

That's something you can optimize for. An overqualified engineer shouldn't take six months to earn their keep unless the codebase is a huge mess. If the overqualified engineer is worth the salary they're really qualified for, they'll be able to describe exactly what parts of the system need to get fixed so the next engineer can be patching code in much less time.

> if it takes 6 months to get a new developer up and running on the codebase

That's the problem you have to tackle as fast as possible. An overqualified candidate would be a good fit there: first task, streamline the onboarding problem. Document, test, document.

Right, but we know there are all kinds of reasons people quit jobs quickly. The question is probably whether someone legitimately "overqualified" is any more likely than anyone else to leave in a particular timeframe.