There have been three, yes three, 13+ year periods - pretty long - over which the S&P 500 underperformed risk-free one-month Treasury bills: 1929-1943; 1966-1982; and 2000-2012.
Will 2013 to 2025 be yet another? If anyone says they know the answer, do not believe them. No one knows until after the period is over. Investing long-term though is not one year or so. Keep perspective.