Richard R. John, a professor of journalism at Columbia, is the author of “Spreading the News: The American Postal System From Franklin to Morse.”

THE Postal Service’s announcement that it plans to end Saturday mail delivery reminds us of its vulnerability to the technological convulsions of the information age. The agency lost nearly $16 billion last year; stopping Saturday delivery, starting in August, would save about $2 billion a year. To preserve the letter of the law, which requires six-day service, the agency would continue Saturday parcel delivery — a shrewd decision, since, thanks to booming e-commerce, the parcel business is one of the few sectors that is actually growing.

Polls suggest that 7 in 10 Americans support the change, but a predictable outcry has emerged from members of Congress, labor unions, periodical publishers and direct-mail marketers. Other critics warn that ceasing Saturday service will be the first step down an irreversible “death spiral.”

The controversy gets at a more fundamental question: Do we want the mail to remain a vital American institution in the digital age?...