Investor keeps 80% in S&P 500 despite warnings of low returns, highlighting the importance of a long-term plan.

Sam Parr holds 80% of his portfolio in the S&P 500 despite Howard Marks' warning that the index's high valuation could lead to low returns over the next decade. Parr argues that while Marks' forecast may be mathematically correct, the opportunity cost of sitting out the market is significant, especially for younger investors who can dollar-cost average through market fluctuations. The S&P 500 includes global giants like NVIDIA and Apple, which justify its valuation. Investors should calculate their required real returns and create a plan to stick with, rather than trying to time the market based on forecasts alone.