One thing his intervention won’t do is fix the problem of mispriced Taylor Swift and Bad Bunny tickets. Companies are forever merging or spinning off units, launching new products and services, discontinuing old ones. Institutional capital is available to entrepreneurs to disrupt even the most established industries. Firms raise prices, lower prices, seek this way and that to attenuate the competitive forces that drive every business relentlessly toward profitlessness. In short, it isn’t 1890 anymore, when the Sherman Antitrust Act became law.

The Journal reported this week on Donald Trump’s questionable intervention in the Live Nation/Ticketmaster antitrust prosecution. The incident recalls Richard Nixon’s 1971 meddling in the ITT case, which, in a remarkable absence of coincidence, resulted in a large donation to Nixon’s re-election effort. As with any subject, when you dig you might reach conclusions surprising to the casual observer. Two Brookings Institution economists broached a taboo subject in 2003. They asked whether antitrust enforcement actually produces public benefit. Their finding was skeptical. It has since been buttressed by an epic run of cases (certainly epic in expense and man-hours) tossed out by federal judges. This litany of defeat includes showy federal efforts to scuttle mergers and acquisitions involving Whole Foods and Wild Oats, AT&T and Time Warner, and Microsoft and Activision.

Every prosecutorial effort nowadays seems to begin with a “market definition” gymnastic, a nadir of self-respect being the Federal Trade Commission’s 2003 lawsuit claiming that “superpremium” ice cream doesn’t compete with, er, ice cream. The case was a flare in the night. A letter writer to this newspaper would much later conclude, “Without the continued threat of antitrust actions, the post-government market value of current federal, antitrust employees would be much lower.”

Could some transaction or arrangement be so anticompetitive as to merit intervention? Yep. Occupational licensing and zoning restrictions, promoted at the local level by organized interests, are two such examples. But this isn’t what we get. We get the kind of case brought by ex-Trump trustbuster Makan Delrahim and current California Attorney General Rob Bonta. Mr. Delrahim showed his quality in the AT&T-Time Warner lawsuit, promptly flopping in court. Mr. Bonta now has essentially revived the same failed argument to block Warner’s merger with Paramount. But the Live Nation case detailed in the Journal is a wonder all its own, a true mess. Most of the gripes against the concert-promoting giant could be resolved with a solution that has nothing to with antitrust. That solution: big-name musical acts simply charging market-clearing prices for their tickets. Antitrust isn’t the answer unless you think Taylor Swift, Bruce Springsteen and Bad Bunny engage in predatory pricing. Yet obviated in an instant would be many aggravations the government now links to Live Nation’s “market power.” Artists are presumed to underprice their tickets because they don’t want to be seen gouging their fans. An alternative explanation some have proposed: Bands prefer a chaotic ticket-selling process because it rewards the most obsessive fan who will spend a fortune on merchandise once inside the show. Mr. Trump, after meeting with Live Nation officials, ended federal participation in the case, but it continues in the hands of 34 state litigants toward an outcome 100% guaranteed to do the public little perceptible good.

Antitrust practitioners operate within a mental framework that overlooks normal prosecutorial discretion, which asks: Does bringing a particular lawsuit offer a net benefit to society? Unfortunately, America doesn’t sunset its laws and agencies. When these run out of legitimate purpose, they become actual menaces. A righteous job of any president is to protect the public from these menaces. Mr. Trump has been a severely mixed bag in this regard. Witness his Feder...