It’s that time again, when news stories about a big case that may end up before the Supreme Court come with a warning that what’s at stake is the “legitimacy of the Roberts court.”

For those not fluent in modern Beltway, let us translate: It’s a threat, aimed at John Roberts. If the chief justice does not produce the desired progressive outcome, the Roberts court will find itself attacked as institutionally illegitimate. In this instance, that outcome is the upholding of Roe v. Wade, the landmark 1973 decision that declared...