To the extent anyone on Wall Street cares—and many will tell you they don’t—records in stocks are good for one thing: advertising. Talk all you want about rates of return or piling it up for retirement, but nothing beats a headline about an all-time high for bringing customers in the door. And in they have come. Cheered by what’s become by some measures the longest bull market on record, U.S. investors have plowed money into U.S. stock exchange-traded funds at a rate of almost $12 billion a month since the start of 2017, five times as much as seven years ago. There are signs of stress—like the recent sell-off in Asia —but so far they appear in U.S. investors’ peripheral vision. Anyone buying stock in an American company right now must be comfortable paying two or three times annual sales per share, a level of shareholder generosity that hasn’t been seen since the dying throes of the dot-com bubble. ...