New Yorkers were horrified last month by smartphone video from the subway that showed a tall, fit man repeatedly kicking an old woman in the head and chest.

“WorldStar that!” he crowed to bystanders, a reference to a website that features footage of brutal real-world confrontations. When the NYPD caught up with the ­alleged attacker, Marc Gomez, there was unanimous applause that the thug would be taken off the street and made to pay for his heinous behavior.

Everyone agrees that Marc Gomez belongs on Rikers. Even if, as he claims, his 78-year-old victim — reportedly mentally ill — had yelled at him and threatened his wife, his response was clearly criminal, excessive and brutal. Jail was made for guys like him.

So it makes you wonder why so many New Yorkers who should know better are agitating to close Rikers, eliminate bail and reduce the number of people who are sent to jail in the first place. Opponents of “mass incarceration” claim that Rikers is full of kids who got picked up for smoking pot or jumping a subway turnstile.

But the reality is that the average person arrested and sent to jail in New York City is just like Gomez: a violent adult with prior arrests, including weapons and (sources told The Post) drug charges.

It may sound hard to believe, but getting into Rikers is almost as hard as getting into an elite college. Just getting arrested won’t land you there. Almost 400,000 people are arrested in New York City each year, but 40 percent of them get their cases disposed of at arraignment, ­either through a guilty plea, dismissal or adjournment in contemplation of dismissal.

Of the people whose cases are continued, 70 percent get released without bail on their own recognizance, based on their promise to return. The actual jail population, which turns over quickly, is now about 8,000.

Of the people in jail on an average day, about 14 percent are serving an actual jail sentence because they’ve been found guilty of a crime. Another 8 percent are parole violators or state prisoners being transferred.

Of the people awaiting trial, about half are facing charges for violent felonies, including murder, manslaughter, rape and assault. The number of people held for misdemeanors is very small. There is, on average, only one person in jail for marijuana possession and two people for turnstile jumping.

Most of the people on Rikers have been there before: Historically, the “previously admitted” rate runs at about 75 percent. A substantial number of admissions to Rikers represent people who have been there multiple times in the same year. The median age of a Rikers inmate is 35; these are people who have demonstrated a commitment to criminality over the long haul.

The movement to close Rikers and replace it with smaller, county-based jails is predicated on the idea that we can easily ­reduce the city’s jail population to below 5,000. But the expectation is unrealistic. We’ve already reduced the number of people arrested and held on drug charges via effective ­decrim­inalization of marijuana, diversion programs and alternatives to incarceration.

Cash bail, according to advocates, amounts to criminalization of poverty and should be eliminated. But many people, even with low bail amounts, are ineligible to leave jail because they have outstanding warrants or holds.

Moreover, New York state law prevents judges from taking public safety into account in setting bonds, a restriction that would likely be lifted if cash bail were abolished in New York. The ­experience of other states, such as California, that have eliminated cash bail shows that judges tend to increase the number of people remanded to jail. Getting rid of bail, in other words, doesn’t necessarily decrease the number of people in jail.

Gomez, who allegedly stomped on an old lady on the subway because, he says, she disrespected him, should fit right in at Rikers. He reportedly already has faced misdemeanor drug raps, as well as an open weapons charge for illegally possessing a shotgun. The disturbing footage of him shows his clear propensity for violence.

Next time you hear politicians or advocates argue for ending mass incarceration, closing Rikers or getting rid of bail, ­remember Gomez — because he’s the guy they want back on the streets, and riding the subway, looking for the next person who doesn’t make room for him quickly enough.

Seth Barron is associate editor at City Journal.