Dan Ariely writes about a meeting with a locksmith:
this locksmith was penalized for getting better at his profession. He was tipped better when he was an apprentice and it took him longer to pick a lock, even though he would oftentimes break the lock! Now that it takes him only a moment, his customers complain that he is overcharging and they don’t tip him. […] Now imagine how much more people would pay if they knew the effort that goes into all kinds of products and services?
Software consultants will probably relate to this. Many “challenging” projects are, in reality, only complicated for less experienced developers. The highly skilled programmer can solve the problem very quickly, but they have to then charge high rates to make up, which many customers may not be willing to pay. Or think of plumbers — they charge a lot for tightening a nut, but because of that, people call them only as a last resort.
The principle here is that beyond a point, the market stops rewarding you for being better at your profession, and if you want to make more money, you need to think of expanding horizontally. This may not even be related to actual skills. For example, I have read that saying that you are a Mensa on your resume (except when you are fresh out of college) has a non-trivial likelihood of getting you rejected because people think you are over-qualified. I am not really sure about the research, but it definitely seems plausible.
So here is a possibility. Maybe the locksmith, instead of being a faster locksmith, uses his fast lock-breaking skills to become a “home security evaluator” who certifies whether the locks in a home can be easily broken. Some rich customers and companies (even lock-making companies) may be willing to pay the big bucks for that.
Many people try to make themselves very skilled in a particular profession. This is okay to some extent as long as the demand in that profession keeps growing. But once supply catches up to demand, or there are low-cost workers who can do the same task with only slight reduction in quality, then having only vertical skills is a big disadvantage.