the s&p (or dow) has had plenty of losing years, lost about 90% in the 1929-32 era and has had plenty of 50%+ drawdowns. if this current bull market were to drop by 50% (i don't think it will), how will you be comforting the poor granny who just lost half her life savings and has no time (literally) to wait for decades for the market to recover?
Yeah, because fully investing poor granny in equities is the way to go...oh boy. Equities allocation is based on age and risk tolerance. A 20 yr old who VALUE cost averages (not DOLLAR) every month until retirement will be better off than 99% of the people here who are throwing their money in swing and day trading.
