As WeWork and its rivals stumble, 18 million square feet of space in NYC is at risk. Here's how that could intensify a real-estate market downturn.

· In recent years, flexible-workspace firms grew dramatically in New York, amassing a nearly 20-million-square-foot portfolio of space that could now result in vacancies.
· Flexible-space firms have had layoffs, sought to restructure and terminate leases, and face an uncertain future as a result of the coronavirus...