I think this just do 1 thing is nice if you just want to do 1 thing. I am now with my 10th language.
Maybe I am mediocre in comparison to some specialists. But at least the specialists rarely complain about me. After the 5th+ language and concept you're getting pretty quick in understanding languages.
And if you stay within the same language family it's even easier. In my opinion most people stay mediocre despite specialization. At some point they decide they know their stuff and just stay there because it's comfortable.
Challenging yourself on a regular basis is exhausting but it can be a lot of fun. And with every language I learn about the other languages as well.
But I get bored easily, and doing LTP (load, transform, persist) on an abstract framework level in a GC based language is easy anyways.
There are a few exceptions that are more challenging prolog for example or the first ML dialect.
Anyways that's just me complaining about these 'do one thing good' attitude. I am more of a free spirit and say 'do what you enjoy'. Everything you enjoy you will get good at anyways ... because you enjoy it, it will come easy.
just 15 minutes a day in another language will get you pretty far within a year 91,25h of coding.
There is a reason why the pragmatic programmer writes: Learn 1 language per year and not master one per year.