The point is that even in the worst-case-scenario, if you bought stock the hour before the great depression, you'd still be way up when you retire in 30 years later.
The fact that stocks go up long run isn’t an argument for pouring money into a plunging market. This might be the bottom, it might not.
I’d rather risk paying more now before a big crash and continuing to invest through the big crash than risk missing this opportunity and the market going back to a bull one in a few weeks.