One day in September, investors with fading summer tans mingled with their brokers over a three-course lunch at Cipriani in Lower Manhattan. Stocks were soaring and they swapped market tips and touted apps that allowed them to buy Bitcoin on the golf course. But the full stomachs and fat portfolios couldn’t mask a sense of unease that pervaded the gathering: that the good times could suddenly end, derailed by nuclear war, political upheaval, a sudden rise in inflation or simply from...