When Home Depot co-founder Ken Langone saw Sen. Bernie Sanders’s rallies on TV during the 2016 presidential campaign, he was shocked by the number of young people there. It troubled him that they were so enthusiastic for higher taxes and spending and an expanded government role in health care, child care and higher education. “If these kids are moving up in our system now, we’re in trouble,” he recalls thinking.

Mr. Langone decided that capitalism needed a full-throated defense. To make the case, he has written a book, part...