Agreed, this is sometimes not usefull. Especially if you thoroughly invest in something you know to diversify in a lot of things "you do not know much about".
As an example in my stock investing, I do much better when I invest in a sector I know will thrive well in the future, and pick my favorite stocks inside that sector / themes.
Lately I have been doing these for e-commerce / new retail stocks (such as Zalando, Hellofresh, delivery hero) and video games (Ubisoft, EA, Activision and smaller publishers). I made 3-4x the return compared to if I had diversified in many sectors I do not believe in (banks, oil and gas, auto, etc...)
That's also another thing I didn't touch on, if you diversify you also move out of your focus area or expertise area and you send up making bad calls. Like for you in equities, you pick companies that you know well and how they compete in that market
But at the same time you could also spin those profits off into another asset class like gold for example, so if the WHOLE equity market takes a hit you have some protection