美國股市目前的估值約為標普500指數預期盈利的22倍，雖然高於歷史平均水平，但仍遠低於1990年代末或2020年的泡沫時期。作者認為這並非泡沫，因為市場由一批極具創新力和盈利能力的企業所驅動，股價並未脫離合理的基本面價值，不同於1920年代或1990年代網路泡沫時期的盲目狂熱。

標普500指數未來的成長高度依賴少數公司的表現。預計僅13家公司將貢獻未來四年標普500營收成長的一半，遠少於前五年的水準。這些公司主要包括「七巨頭」（扣除特斯拉）、人工智慧相關企業如博通、美光、甲骨文和戴爾，以及福特、沃爾瑪和麥克森等。同時，利潤率已從1990年代初的4%飆升至15%，進一步擴張的空間有限，因此營收成長成為推動盈利增長的關鍵。

市場集中度的風險已在2026年初顯現：剔除七巨頭後，標普500年初至今上漲約13%，優於含七巨頭的整體指數表現約3個百分點，而七巨頭自身僅錄得約3%的漲幅。作者總結認為，標普500的問題並非估值過高得離譜，而是持續成長的命運掌握在越來越少的公司手中，一旦這些公司令人失望，市場的高速增長時代可能就此終結。

The US stock market is not in a bubble, according to the author, because its valuation of roughly 22 times forward earnings for the S&P 500 is driven by highly innovative and profitable companies rather than irrational speculation. While the premium is higher than usual, it remains well below the extremes seen in the late 1990s dot-com era or even in 2020, and the underlying businesses represent some of the most impactful enterprises in history.

Future growth for the S&P 500 increasingly depends on a narrow group of companies. Just 13 firms are projected to generate half of the index's revenue growth over the next four years, down sharply from the previous period. These include most of the Magnificent Seven (excluding Tesla), AI-focused companies like Broadcom, Micron, Oracle, and Dell, along with Ford, Walmart, and McKesson. Meanwhile, profit margins have already surged from 4% to 15% since the early 1990s, leaving limited room for further expansion and placing the burden squarely on revenue growth.

The risks of market concentration are already visible in 2026: the S&P 500 excluding the Magnificent Seven has gained about 13% year-to-date, outperforming the full index by roughly 3 percentage points, while the Magnificent Seven themselves have managed only a 3% return. The author concludes that the core concern is not that the S&P 500 is dangerously overvalued, but that its continued growth rests on an ever-shrinking set of companies—if they fall short of lofty expectations, the market's era of rapid gains could come to an end.