I guess the market analogy would be that you want to buy into an index. So if you’re invested in PHP but you see over 20% of jobs say React it might be time to diversify a bit into React.

This leaves out that diversifying one’s tech skills isn’t as easy as diversifying a portfolio though. You could do a React side project, but it won’t be considered as highly as a full time job. I guess that’s the difference between adding a bit to your portfolio vs. going all in.