Interesting. I think there are more factors at play here. Learning a new language has a definite "cognitive cost". One issue is the perception of banality/drudgery i.e. lack of excitement/motivation involved in learning a language which is not too different (computation modelwise) from what you already know. As an example, having worked with C/C++ for a long time i just cannot summon the motivation to learn Python/JS etc. They are "boring". The constant comparison of features/syntactic sugar soon becomes tiresome. And subconsciously you feel that the effort/payoff ratio is not worth it until external circumstances force a change (eg. a new project requiring the language).

I think it is better to be fluent in one or two languages so that you can focus on the problem without bothering about other incidental complexities. The only time where the effort in learning a new language is worth it is when you learn a new model of computation/thinking that expands your mind (eg. Erlang, Haskell, Mozart/Oz)