You don't think people should skim off some of their profits into fiat to ensure that they don't ride it all the way up and all the way down?
I worked for a company during the dot com boom. Our stock went from $2.00 to $204 in less than a year. I took a big chunk of profits along the way. Many of my co-workers believed the hype and rode it all the way up, and all the way back down to pennies per share, sure the stock would eventually turn and make them all multi-millionaires.
Knowing when to sell is really hard.
You have a good point, you need at some point diversify and make sure to have liquid money available in case of a market crash!