Every generation has its market giants, companies that seem unstoppable. For decades, IBM held that title. Today, the spotlight shines on the so-called “Magnificent 7”: Apple, Microsoft, Nvidia, Amazon, Alphabet, Meta, and Tesla. But history reminds us that even the strongest companies can lose their edge over time. In this video, we look at how IBM’s long, gradual decline offers a lesson for investors who might be tempted to concentrate too heavily in today’s market leaders.

Since 1926, the US stock market has rewarded investors with an annualized return of about 10%. But returns in any given year may...