DECLINE IS A CHOICE: A fare-beater crackdown is just what New York needs.

Today, Manhattan District Attorney Cyrus Vance Jr.’s decision not to prosecute subway-fare theft threatens to undo the gains. When he announced the new policy in February, Vance touted it with lofty rhetoric about “civil, rational enforcement.” Declining to take farebeaters to court, he said, would save money, promote fairness and proportionality, and improve safety by allowing cops to fry bigger fish.

But the evidence from the first nine months is already casting a pall on these sunny predictions — and foreshadowing a return of the bad old days underground.

Earlier this month, NYC Transit, the division of the Metropolitan Transportation Authority that oversees the subways, revealed that it had lost $215 million this year to “fare evasion.” That’s up from $105 million in 2015 and amounts to 88 percent of the MTA’s projected budget deficit for 2019.

Yes, the data came in the form of a preliminary estimate, as a spokesman for the DA was quick to emphasize. But the spike was high enough to suggest that farebeating in Manhattan under Vance’s lenient regime is at least a significant factor in the losses, as NYC Transit boss Andy Byford told the City Council.

Michael Bloomberg keeping most of Rudy Giuliani’s “Broken Windows” crime policies in place made New York livable again. Naturally, Bill DeBlasio wants a return to the bad old days.