In an article for Bloomberg, columnist Nir Kaissar discusses the quandary faced by many money managers who have seen the S&P 500 outperform other investments over the last decade.

“No diversified portfolio has any hope of keeping up with the market, not even those belonging to elite universities,” writes Kaissar, adding, “Most stock pickers can’t keep up, either.” Therein lies the quandary, he asserts, since U.S. investors “care a lot about keeping up with the S&P 500.” He cites arguments made by market legends such as Warren Buffett and Vanguard founder John Bogle, “who have said that most people need little else in their portfolios.”

That said, Kaissar argues, “there are good reasons why managers don’t dump everything and chase the S&P 500. The most obvious is diversification: They are more likely to achieve better risk-adjusted results over time by spreading their bets across different types of investments.”

The article also notes that U.S. stocks are expensive, but Kaissar says, “this doesn’t mean the market can’t keep rising, of course,” citing recent comments by Robert Shiller that the market could move “a lot higher before it comes down.”

Many investors have left their money managers with what Kaissar describes as “two unappealing choices: abandon their strategy and chase the market, or watch investors walk out the door.” He concludes, “This bull market isn’t likely to last forever,” but in the meantime, “managers may increasingly have to choose between their convictions and their clients.”