It’s a random Saturday in May, and I can barely find enough free space inside Allbirds’s Soho store to try on a pair of what the brand proclaims the “world’s most comfortable shoes.” I’ve been on rush-hour subway cars filled with fewer people. I have to ask a harried sales associate twice to get the $95 Wool Runner sneakers in my size. While waiting, I fondle a display shoe that looks like a pared-down version of an Adidas Yeezy Boost 350. Yet, while that shoe’s upper is made from a synthetic material, the sneaker in my hand is knit from merino wool, making it resemble a winter sock affixed to a white rubber sole, with...