The problem is that coronavirus escaped and is now circulating worldwide. So as soon as they drop the quarantines, they're likely to end up reinfected from some other country. Most of the infections in the U.S. are now coming from Italy and Iran, not China. All it takes is one asymptomatic carrier (and evidence from South Korea and the Diamond Princess indicates that close to half of cases are asymptomatic) to start a local outbreak. The Washington State and Italian outbreaks were both started from a single undetected index case, for example (Washington State was the original index case from Wuhan in Seattle, Italy was traced back to the German car manufacture that had a local outbreak in early January).

You can't really stop a highly-infectious airborne disease with no natural immunity in the population. The best you can do is buy time to develop treatments or a vaccine, or hope that it mutates into something less virulent.