Here's a little thought experiment to get you steamed: What if the celebrated urban planner Edmund Bacon had embraced the prevailing ideology of the 1960s and leveled Society Hill, replacing its blocks of outmoded, colonial-era townhouses with sleek modern high-rises for middle-class families? Would Philadelphia be a livelier, more successful place today?

Frankly, it's hard to imagine that wiping out one of today's most desirable urban neighborhoods in the city, if not the country, could have benefited anyone, rich or poor. In the best case, Society Hill might have become a boring, middle-class enclave similar to New York's Upper East Side. I'd rather not think about the worst case.

The story of what happened in Society Hill has long been regarded by planners as a watershed moment for American cities. By renovating its dilapidated housing stock, Bacon demonstrated that historic preservation could be a powerful economic-development tool, one that has guided Philadelphia's slow but steady revival for half a century.

That narrative is now being challenged in some unlikely intellectual corners. Rather than helping our cities recover their bearings, historic preservation is strangling them, the revisionists assert. They blame our sentimental affection for old buildings for everything from sky-high rents to the economic whupping the United States has taken from China.

This argument comes not from a lone crank, but two of the most respected thinkers in their fields: Pritzker Prize-winning architect Rem Koolhaas and Harvard University economist Edward Glaeser.

These are not two guys you would normally expect to find at the same party. Koolhaas is the cooler-than-cool, leftist provocateur best known for designing the Seattle Public Library and the Prada "Epicenter" in New York's SoHo. Glaeser, who will address the Philadelphia chapter of the Urban Land Institute June 14 at the Union League, is a wonky free-marketeer who just published an unlikely celebration of urban centers called Triumph of the City.

Overall, Glaeser's book is a welcome corrective to Robert Bruegmann's 2005 free-market defense of sprawl. Glaeser, by contrast, believes cities have a built-in competitive advantage because their dense, diverse concentrations of people create the perfect ecosystem for generating new ideas and new businesses, and doing it in the most efficient way possible.

But Glaeser goes overboard with his emphasis on efficiency, which, like many economists, he sees as the ruling determinant in the universe. Since close proximity increases the frequency of serendipitous exchanges, he concludes that cities should cultivate maximum density and allow as many skyscrapers as the market will bear.

More high-rises, he says, translates into greater supply and lower costs. If housing in Philadelphia were as cheap as in Houston, Glaeser suggests, this city would really rock. Smart people would be drawn here because rent was such a bargain, and they would create businesses and jobs that would make Philadelphia wealthy again. If only!

Just look at China, Glaeser insists. They can't erect skyscrapers fast enough. And isn't China booming?

The problem is the darn preservationists won't let American cities behave like Shenzhen. Places like Philadelphia are so irrationally attached to their old, low-rise, inefficient rowhouses that they protect them with a Byzantine web of preservation laws. The occupants of those fine Society Hill houses are effectively keeping prices high for everyone else, forcing people to seek housing and work in less expensive places.

It's funny to hear Koolhaas echo this free-market analysis. Like Glaeser, the Dutch-born architect has a natural affinity for cities, which he championed in Delirious New York, the 1978 manifesto that was one of the first books to appreciate the messy vitality you get in less-than-perfect urban conditions. Koolhaas has always positioned himself as a critic of capitalism (even while taking commissions from some of its biggest practitioners) and its ramped-up consumer culture.

But Koolhaas also has had several projects thwarted by a powerful group of anti-consumers: preservationists. His proposal for expanding the Whitney Museum of Art in New York was checked after an uproar over four adjacent historic townhouses, and he lost the commission for London's Tate Modern to competitors who preserved an old power plant. These grudges heavily inform his show "Cronocaos" at the New Museum, located on a rapidly gentrifying stretch of the Bowery in Lower Manhattan - not far, as it turns out, from his Prada store.

Instead of staging the show, which runs through Sunday, in the museum, Koolhaas sets up his polemic in a former restaurant-supply store next door. Half the room has been left untouched, with torn patches of fake-brick vinyl flooring and framed certificates from industry associations. The other half is given the white-walled gallery treatment, so we can witness gentrification in progress. Koolhaas gets to condemn the cycle even as he aids and abets it.

The thesis of Koolhaas' rant is that the world is being frozen in amber by a powerful heritage mafia, which he suggests is one reason Europe and America are falling behind China - as if its lack of concern for ancestral hutongs explains its growing economic might.

Occasionally, Koolhaas hits a bull's-eye, as when he complains that preservationists force architects to design in historically friendly, but fake-looking, traditional styles. He's also right that excessive zeal for historical accuracy can lead to a form of architectural cleansing that costs neighborhoods their grit and authenticity. The show gets its name, "Cronocaos," from Koolhaas' cheeky conflation of the words chronological chaos, his phrase for the excessive worship of civilization's past glories.

Even as they decry totalitarian preservationists, both Koolhaas and Glaeser seek to impose a one-size-fits-all approach to cities. Skyscrapers have their place, and Philadelphia could do with a few more. But tall buildings aren't the only way to achieve high density. With 11 people to the acre, Philadelphia holds its own with parts of New York. A neighborhood of tightly packed rowhouses is a far more attractive place to live than one made up of widely spaced high-rises - like Koolhaas' CCTV tower in Beijing, almost a mile from its nearest neighbor.

I also wonder if Glaeser knows that most of the Philadelphia condo towers built in the boom decade now sit half empty, while individual rowhouses continue to be built and sold. Or that the city has no shortage of vacant land for affordable new housing.

It's no accident that Philadelphia's strongest neighborhoods are those with the most intact historic fabric. The city's comeback has been built on old foundations. That, more than cheap high-rises, is what will make people want to live here.