It's not a black and white issue. "Jack of all trades, master of none" vs "expert of one". Both have their place and I think it's better to have a mix than just one or the other.

I've seen python newcomers writing code as if they were writing in another language. They don't know about dataclasses, operator overrides, __init__ vs __new__, metaclasses, __init__.py vs __main__.py, @property, match, the walrus operator, or assignments, or the common pitfalls of python like mutable defaults, type hints, and a bunch of other things.
Knowing a language in-depth helps write DRY code, avoiding common pitfalls, handling things better like debugging, profiling, and other tooling, and avoiding pitfalls of the language, which newcomers have to first learn, regardless of how their experience with other languages.

A lot of stuff is transferable, for sure, but every language uses different idioms, covers different paradigms, and so on. It's good to have at least one expert on the term to teach others, and to have people flexible enough to switch of willing to learn. Having only experts can mean a static team unwilling to experiment or use better programming languages or technologies. Having only beginners or mediors of a language can produce functional, but sub-optimal code. YMMV

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