This strikes me as completely the wrong way to make decisions. If you're taking it as a given that you're a top performer in a common field, then yeah. You might as well assume you're a billionaire and argue how many doors that opens for you.

But you're not a top performer in a common field until you've committed years of effort to not being a top performer in that field. And you can hardly argue that the world is a worse place because someone wanted to be the world's best yodeler instead of the best truck driver.

What made me think that functional programming is worth being aware of is that good testing practices head towards things done more easily in functional programming, pretty quickly. Programming in functions guarantee a compartmentalization of state and a determinism of the result (as opposed to non-determinism).

Procedural programs and Object-Oriented programs need to postulate concepts like design-by-contract, assertions with explicit precondition/post-condition checks, and unit testing suites as a form of insurance against non-determinism and poor compartmentalization. So it's possible to get this kind of control but only after writing a lot more code.

Going completely stateless is probably foolish though.

As for your heuristic: yes, I agree, a top performer in a common field is a safer bet and a sound strategy. If you're able to secure yourself properly you won't be overcome by a large increase of top performers, if such a thing is possible. We have a lawyer over-supply so I guess in some fields it is.

You don't have to be an "X" developer. Developers can know more than one language, and knowing functional programming can definitely be a competitive advantage, even if you don't typically use functional languages.