The author isn't suggesting that this is the way to achieve true happiness. They are suggesting that this is a good strategy to become the best at something, regardless of why you want to be the best at it (financial benefit, personal fulfillment, etc.).
> More so, what happens if industry trends shift and it turns out you specialized on the 'wrong' thing
What's the alternative to this? Remain mediocre at a number of skills just in case one of them go out of style? If you want to be the best at something, specialize in that thing but keep a pulse on the direction the field is moving and adapt as needed. The world's best mainframe developer probably won't immediately be the world's best PC developer upon switching, but they will be far from starting from square one.