I think you’ve missed the real point of the article. Yes, of course it is suggesting that the wealthy should invest across a diverse set of asset classes, and

I think you’ve missed the real point of the article. Yes, of course it is suggesting that the wealthy should invest across a diverse set of asset classes, and within those asset classes they should have diverse investments: an index fund would be great for the part of their wealth put into stocks. But the point is that if you want intergenerational wealth and you want it to grow essentially forever, your extremely wealthy families must be willing to drastically reduce spending when their portfolios are doing poorly. For decades at a time, if necessary. Sell the yacht, sell the mansion, all so that your great grandchildren (who you will never know) can be ludicrously rich. It’s no wonder few have done so.