Indeed, one does not always need to learn new tools when available tools suffice; the learning energy can be more effectively spent elsewhere.
I only meant to notice that there's no impenetrable barrier to mastery of that, when desired. I don't think that to learn Haskell at a high level is harder than to do the same with C++. The latter just had more adoption for last 25 years, for a number of reasons.
In any case, some (cursory) knowledge of Haskell improved my Java, Python, and JavaScript quite a bit, and is directly applicable to Scala in many aspects.
Somewhere down closer to #15 is "learn Haskell," which comes after learning Rust/Go. Learning a "real" functional language would be much higher, but I already did that early in my career.