President Trump’s famous slogan, “Make America Great Again,” was always an insult to the nation. Isn’t it great now? MAGA harks back wistfully to a presumptively better time when America was a whiter nation with fewer immigrants, when few women held positions of power, and when it traded less with other nations. That’s an image of the 1950s or early ’60s.

Part of the Trumpian urge to recreate past “greatness” takes the country to extremely dark places. But I’ll stick to the economics. Let’s imagine—yes, it’s a flight of fantasy—that...