Most of what was good in the American economy last year was unsustainable, and most of what was sustainable was not good. A decade after the financial crisis, there is still no sign the economy can generate the consistent growth of 3% a year many continue to hope for. The growth rate for 2017 was just 2.5%, and even that seems unlikely to last. Is this the new normal?

Not exactly. Instead it’s a return to the old normal, a reversion that was widely expected after baby boomers began to retire. While policy makers should do what...