Most of us cannot be those "greatest minds that have thus far graced this planet." For many of us, it's more fulfilling to be good at one thing than mediocre at dozens.
And often they’re really good at the one thing because they have their hands in many things.
Cultivating a breadth of knowledge often isn’t just being mediocre in a bunch of subjects, but about broadening one’s awareness to potential connections and opportunities.
If you can’t go deep on anything, that shallow knowledge may not get you very far.
But if you can go deep on X subject with enough surface level knowledge of a variety of potentially related topics, you’ll be able to accomplish far more interesting things than someone with equivalent but exclusive knowledge of X.
My bosses task me with being the 'dot connector' for a bunch of specialists. Seems to work out well.
If you really want to know a subject well beyond theater, you obviously would have to know many adjacent subjects and many seemingly unrelated subjects since "subjects" in this context are just imaginary divisions within human knowledge.
If your interest is credentialism though then the unrelated subjects are just a complete waste of time and most likely the adjacent subjects are just a distraction from memorizing your lines for the theatrical "subject matter expert" performance.
It is really a question of the best way to pretend to be a great mind when you are average and that is to specialize to a degree that minimizes the amount of people who know what you are talking about on the subject of interest.
We have built an entire class of society on this method.