Big Tech’s Market Dominance Stirs Debate on Concentration Risks

The unprecedented level of concentration in a handful of tech stocks provokes strong feelings on Wall Street.

Some investors fear the top-heaviness means the bull run is on quicksand and will sink the minute an AI-frenzy-fueled Nvidia Corp. or Microsoft Corp. stumbles. The dominance also leads to portfolio risks, with diversification suffering. For others, the worries are overblown, gains have historically been powered by a small group of megacaps, and this is simply tech’s latest time to shine.