With the close of the year, the tally was in: Crime was down in the 30 largest cities in the United States, and even a worrisome uptick in urban murders had subsided.

More than two decades of safer cities has cleared the way for major changes in the nation’s criminal justice system: fewer prisoners, shorter sentences and more pardons.

But fewer crimes have not resulted in fewer police officers on the streets.

In 2016, there were slightly more officers per capita than in 1991, when violent crime peaked, according to data collected by the F.B.I. Now, officers deal with half the crimes per capita that they did then.

But hardly anyone questions the size of police forces. Not taxpayers, who might expect the decades-long drop in crime to produce some budget savings. Not politicians, though they have a host of competing priorities, like schools and hospitals.