The question is not whether the average investor can

"do better" by diversifying; the question is whether the average investor can find a single manager to do the diversification process at a reasonable price. In February of 1981, Berkshire Hathaway was trading at $430 per share; today it trades at $680k per share. BRK-A is fundamentally a buy-and-hold diversification strategy linked to a positive cash flow resulting from prudent insurance underwriting.

I was late to the BRK-A story. But, i bit the bullet in 1998 and never looked back. Ten years later, I bought Markel Insurance Company (MKL) -- although this time at a price closer to BRK's in 1981. When you compare the long-term price appreciation of both assets, the similarity is amazing. Is MKL worth it today at $1500+ per share? Yes, if the price get close 1.1x its book value per share. Does it happen often? No, but it does happen.

Here's the real challenge: do you have the fortitude to do nothing for 44 years (if you bought BRK in 1981) or 25 years if you bought Markel in 1999? Charlie Munger used to say that BRK was really nothing more than 40 +/- good decisions over 60+ years. The insight we need to have is not just seeing the value of diversification but also the importance of time in accurately assessing investment success.