Urban metros, though beloved by millennials and increasingly a favorite of affluent baby boomers in search of luxury senior housing, are vulnerable to future economic downturns, a Bloomberg expert said.

During the Great Recession, cities were the best performers — not surprising since the housing crisis was focused in suburban and rural markets. But Bloomberg's Conor Sen wrote that city centers now are inundated with specialized high-end jobs in the tech and financial services spheres. This concentration of wealth in urban metros, which today is among their greatest asset, could be their undoing, Bloomberg reports.

“Cities today increasingly resemble endless aisles of Champagne and cupcakes. If tough times strike again — perhaps in a tech downturn, or in a stock market crash — the pain will be concentrated here. And while the well-paying white-collar jobs migrating to cities now are converted, there’s no guarantee the best jobs will always be urban. The next economic cycle may well bring a different pattern,” Sen wrote in Bloomberg View.

Though there is another economic downturn on the horizon, no one can pinpoint when it will occur. The country is experiencing one of its longest economic expansions on record, closing in on the 10-year record held from 1991 through 2001.

The long length of the cycle is beginning to rear its head in commercial real estate. Though experts say there is still room to grow, investors are proceeding with caution due to the inflated state of property prices, deal volume has cooled nationwide and foreign investors are slowly turning away from U.S. gateway markets in search of better risk-adjusted returns.