Consumers today can get food and rides and tubes of toothpaste on demand at any hour of the day. They can order used cars, up for sale, to be brought to their doorstep for a test drive in mere hours. Startups—clamoring to cater to consumers’ desire for all things white-glove and personalized—have encouraged new users to outsource their laundry, their driving, their parking, their shopping, and more. But even the business leaders who are helping to revolutionize today’s consumer lifestyle disagree about whether this is a golden age that can last—or one built on gre...