Scott Pape, known as the Barefoot Investor, has cautioned Australians against withdrawing their superannuation and share portfolios from index funds due to fears of a stock market crash. His advice comes in response to concerns raised by a reader who cited billionaire investor Jeremy Grantham's predictions of an impending downturn in U.S. stocks.

Grantham, who manages approximately $78 billion, suggested reducing exposure to U.S. equities due to their perceived overpricing and shifting investments toward emerging markets and bonds. However, Pape urged investors to be skeptical of making significant changes to their long-term investment strategies based on market forecasts. "History shows shares deliver the best long-term returns of all investments, even though they scare the living daylights out of you sometimes," Pape stated.

Pape emphasized that attempting to time market downturns often harms long-term investment performance, noting that every market crash has ultimately been followed by new highs. He likened abandoning a long-term strategy to making rash decisions in a marriage, suggesting that such advice is designed to create restlessness and encourage a shift away from stable index funds toward potentially riskier assets.

While acknowledging that Grantham's predictions might prove correct, Pape highlighted the difficulty of successfully timing the market. "To make money from that, you've got to be right twice. First, you've got to sell before everyone else. Then you've got to decide when the coast is clear and buy back in," he explained.

Pape pointed out that Grantham has repeatedly warned about U.S. shares being in bubble territory since 2021, a period during which the American market has seen substantial growth, with the market climbing more than 100 percent higher. "Even Grantham has struggled with this," Pape commented.

For investors, particularly those like the 42-year-old reader who have decades until retirement, Pape stressed the importance of time as a key advantage. "At 42 you've got your greatest advantage: time," he wrote. "Every market wobble is a chance for those dollars to buy more shares and compound."

Pape's core message is to remain invested in diversified funds, avoid emotional decisions driven by dire predictions, and maintain a cash reserve to prevent being forced to sell during a market downturn. "Stay married to your diversified share portfolio," he advised. "Keep some cash aside so a crash never forces you to sell."

In his interview on the Diary of a CEO podcast, Grantham expressed low confidence in the outlook for American shares over the next decade, stating, "I'm not confident that US equities will be intact in five years or 10 years. Because they're so badly overpriced today." He drew a parallel to the tech bubble of 2000, where U.S. equities experienced negative returns for a decade, and believes the current market is priced even higher.