Much of this scorn is deserved. And those who shrug should take a second look. In one recent comment about this tale of two cities, a reader here wrote: “Why is it a problem that underachievers are moving out?”

Why? Let me count the ways. A city without its nurses, its teachers, its artists, its waiters, its bus drivers, its cops, its musicians and writers and grandmothers as residents is a monoculture — as sterile as a forest of a single commercial tree species.

But much of the scorn is also not fair. Cities develop organically, and if the young, rich and digitally obsessed want to cluster in this peninsula by the Pacific, who’s to stop them. A neighborhood like the Mission, with its beautifully tattooed buildings, open-air bodegas and Victorian flats unchanged since Jack London’s day, is a place close to my heart. But it’s not destined, for all time, to be frozen.

Go to any city with a thriving hub of young, creative job holders and you’ll find a version of what’s happening here. New York has its Park Slope; Portland, Ore., has its Pearl District; Seattle its South Lake Union, all thick with people who work at jobs that don’t pollute, pay well and tap into the tomorrow economy. Detroit, Cleveland or El Paso would sell their civic souls for a shot at some of that scorned prosperity.

Some solutions — a raise in the minimum wage, workable rent control, more housing for the middle class subsidized by taxes on luxury high-rises — are getting a fresh airing. The United States now ranks third among all advanced nations in the amount of income inequality, according to the Stanford Center on Poverty and Inequality. This issue is at the heart of preserving our nation’s egalitarian sense of self.

But absent a big legislative resolution, there is one thing a city like San Francisco can do to hold onto some of its unique character (and characters). That brings me back to my ruined evening, courtesy of Caltrain. The next day, BART, which carries 400,000 passengers a day on its 104 miles of track, broke down in the morning rush hour, stranding thousands of people trying to get to work. The Muni system, which operates the city’s buses, is the slowest fleet in the country, averaging eight miles an hour; its breakdowns cost the city $50 million a year in lost productivity.