Even if you know the next ten years will be in the bottom decile of returns for the S&P 500, you’re still better off than with cash.
"Suppose I offer you a security that will pay $100 two days from today. You can buy as much of it as you like today at $50, or you can wait until tomorrow. Tomorrow, I’ll flip a coin. If it’s heads, I’ll sell you the security at $99. If it’s tails, I’ll sell you the security at $25. What should you do?
Clearly, if you buy the security today, you’ll double your money two days from now. That’s a 100% expected return for each dollar you invest, over that 2-day period. If you wait, you’ll earn nothing on the first day, but you’ll then have two possibilities. If heads, you’ll get just 1% on your invested money. If tails, you’ll get a 300% return, quadrupling your money. With a 50/50 chance at each, your expected return for every dollar you invest is 0.51% + 0.5300% = 150.5%. So waiting adds 50.5% to your expected return over that 2-day period."
"the bottom decile of returns for the S&P 500" I guess we have not yet seen what that means. The next ten years might show negative yearly returns.