“As a final note, with respect to the very largest glamour stocks, my impression is that investors should consider the opposite implications of “market share” arithmetic. Presently, the largest 5 stocks in the S&P 500 comprise about 23% of the market capitalization of the index, easily eclipsing the 2000 peak, when the big 5 represented just over 16% of index capitalization. Here’s the thing. Investors now require these stocks to maintain their current market share, relative to the index as a whole.
Consider a bear market that brings valuations only to 2002 levels – the highest level of valuation observed at the end of any market cycle. If the S&P 500 was to decline by 50% overall, and the largest 5 stocks moved back to the 2000 extreme of 16% of the index (which they represented as recently as 2019), the implied loss for these stocks would be 0.5(.16/.23)-1 = -65%, while the implied loss for the rest of the S&P 500 would be 0.5(.84/.77)-1 = -45%. That’s not so much a projection as an implication of arithmetic, so we should at least consider this possibility given that the market cap of the largest 5 stocks has become unusually skewed.“
John Hussman, Market Commentary August 2020