The coronavirus pandemic will eventually be behind us, maybe soon, maybe not. Either way, the scars will long persist for an entire generation of investors and future managers now living through what is rapidly becoming the deepest downturn since World War II.

Investors in their formative years have just discovered that stocks come with big risks attached. Unless there is a rapid recovery—the V-shaped economic bounce many are hoping for—these investors could take 2020 as their reference point for bad news and have a lower...