That would only make sense if McKinsey's main business was investing in the stock market. Actually their main business is selling management consulting, which a

That would only make sense if McKinsey's main business was investing in the stock market. Actually their main business is selling management consulting, which as a source of profit is only weakly correlated to investment returns. Also note that they didn't say you should expect a negative return from investing in the stock market. In fact, they said that you should expect a 4-6% return over the next 20 years, rather than the 7-8% return that has been experienced over the previous 30 years.