The current share of the S&P 500 market capitalization held by the largest five firms is a staggering 25%. Those firms are, in order of market capitalization, Microsoft, Apple, Alphabet, Amazon, and Nvidia. The last time the top five were collectively this large was the early 1970s. At that time the five firms in market capitalization order were IBM, AT&T, GM, Eastman Kodak, and Exxon. No firm is unassailable.