This notion of passive investing that has been pounded into peoples heads for years is complete bullshit and has only worked because there was always someone else ready to pay more for the same asset and because rates were perpetually held low. Some points to consider:
(1) You have fewer millennials than baby boomers, as the baby boomers cash out from their vanguard accounts, who makes up for the difference?
(2) If the S&P 500 contains companies built for a certain macro regime (low inflation, low interest rates), and the macro regime is shifting, you can be penalized by owning a set of assets that do not provide adequate returns (Tesla is currently the 5th largest weighting in the S&P, they don't pay you squat.)
Go look at charts of the S&P 500 beyond the last 40 years when rates were more variable, you'll see the market can at times be a shit investment vehicle that might not give you a return by the time you retire and on an inflation adjusted basis has a negative return!
Buy low, sell high. Save cash and be patient.