2008 to today is eleven years. Please look at the immediately previous eleven-year period. The 2008 crisis left the S&P down compared to where it had been in 1997. How cheerful would you be about retirement if your own 401(k) returns had been negative over the last eleven years. What if a 2008-like event happened just before you had planned to retire, or just before/after other major financial events such as buying/selling a house or kids hitting college or someone getting sick?

It's great that your prime savings years have coincided with an exceptional run-up. I also have been fortunate. The difference seems to be that I don't look down my nose people who lacked perfect flexibility wrt when to enter or leave the market. I have reasons to believe I've been pretty smart about my savings and investments, but I still admit the role that pure luck has played.