A funny thing happened in the half century since Jane Jacobs published her classic treatise excoriating the planning establishment for clear-cutting American cities and replacing eclectic neighborhoods with sterile housing towers: Her vision of urban change won the day. From Brooklyn’s Clinton Hill to Philadelphia’s Society Hill, the neighborhoods that have revived according to Jacobs’ principles became not merely livable, but immensely desirable.

The trouble is, that vision is also giving us a new kind of sterility.

Jacobs, whose The Death and Life of Great American Cities serves as the bible for city-lovers and modern planners, believed that blighted neighborhoods would regenerate organically if left to their own devices. Existing residents would fix up their homes as their economic circumstances improved over time. Drawn by the charms of these diverse, lived-in neighborhoods, newcomers would migrate in gradually to refresh the mix. The rundown districts would, in Jacobs’s lovely phrase, naturally “unslum.”

Today, virtually every older city, save for some unlucky hard cases, can boast at least one turn-around story. U Street in D.C., Lodo in Denver, Highland Park in Pittsburgh. You see the twenty-first century version of Jacobs’s beloved “street ballet” playing out today among the renovated rowhouses in Philadelphia’s Graduate Hospital section, where only a decade or so ago gunshots provided the beat in the background noise: People leading pets to dog parks, picking up Italian kale at the corner farmers’ market, meeting friends at the local gastropub, admiring the latest yarn-bombed bike rack. The only housing towers going up in these rising neighborhoods have penthouses and lap pools.

But Jacobs’s predictions of multi-generational, multi-race, mixed-income kumbaya hasn’t turned out quite as she hoped. “Unslumming,” she wrote back in 1961, “hinges on the retention of a very considerable part of the slum population within a slum.” Unfortunately, that rarely happens. Today we know the process she described by another name entirely: It’s not unslumming. It’s gentrification, a word that doesn’t sound nearly as quaint or benign. It’s worth noting that the term didn’t come into use until a full three years after the publication of Death and Life, when it was coined by the British sociologist Ruth Glass. Appealing as it sounds in theory, Jacobs’s picture of hard-working locals hammering and spackling their way to an unslummed paradise has proved more romanticized than real.