I had been afraid to check our portfolio, assuming we were approaching "penniless" status by now. But yesterday I logged in and found, to my surprise, we'd lost only 4.7% of the total currently invested with this particular brokerage from 2/28 to 3/29. Mr. M's cousin, a retired investment advisor, has been buying stocks throughout this Corona frenzy, and we've taken his advice.
A well-balanced 401K should show the drops you mentioned, because they should be diversified among different funds. In my case, the mix was something I had control over, and adjusted my risks to match my situation. Luck came into it when I had to change my balance upon retiring unexpectedly. The market was in a good place when I closed it out.
ETA: I saw losses in the neighborhood of what you lost in two previous dumps: The dot-com bust and 2007-08 panic.