People have been greedy for a long time.
"In 1929, at the height of an economic boom in America, Joseph Kennedy Sr. (father of JFK) was working as a stockbroker on Wall Street. As the story goes, Joseph was walking around when he decided to sit down for a shoeshine. While polishing his shoes, the young worker gave Joseph some of his favorite stock picks. When Joseph heard the shoeshine boy giving out stock tips, he figured the party was about to end, and it was time to get out of the market. Joseph proceeded to exit his positions in the market and bought short positions that bet on the market going down."
https://www.pitzlfinancial.com/blog/ode-shoeshine-boy
We're not going to have a 1929 style crash, and we might not have one at all, but a 5-10 percent downturn, or even larger, wouldn't surprise anyone.