Statistics like this ignore one aspect: how well can you understand and follow up 60+ different companies? ... If you are highly involved, concentration is a benefit. If not, it is a liability and you should stay away, and go for a broad index. ...
As usual I agree with you, mostly.
The OP's premise (I think) was that he/she was concerned about owning an individual stock. So I accepted that premise.
My point in dragging Markowitz in was that, if the OP's goal in managing stock concentration was diversification, he/she would have to hold many stocks. Too many stocks to be practical, as you point out. From that I expected the OP to conclude that individual stocks was not the path to diversification.
But, hey, give old Harry a break. His 1952 paper was totally theoretical and mathematical, with no concern for practicality. Also, the paper predated Wells Fargo's invention of the index fund and Bogle's commercialization of the idea by close to 25 years. So Harry was pretty much on a different planet.
I do disagree, though that, in the context of individual stock picking, concentration is a benefit. I guess it is a benefit if you are seeking to self-medicate with occasional dopamine shots (Jason Zweig:
Your Money and Your Brain) but the statistics say that it is highly unlikely to be beneficial to financial success over the long term. I was talking to a TDAmeritrade branch manager one time and I asked her what their aggregate individual stock traders' results were for the previous year, 2016. "1.5%" was the answer. Of course the Dow was up 13% that year and other indices were up anywhere from 5% (ACWI) to 25%(S&P Small Cap). Worse, I doubt that the 1.5% included consideration of survivorship bias. I don't know how it could have. As Charles Ellis points out, stock picking is a loser's game.