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.
It has infected <0.1% of South Korea's population and the number of new cases per day has been going down for several days in a row now. Today they reported less than a hundred new cases.
It sure doesn't seem to be sweeping through the South Korea population like wildfire. It doesn't appear they will even reach 1% infected, let alone 30-60%.
It also doesn't look to be on a path to infecting 30-60% of China's population. New infections in China have dwindled to almost nothing, and the total number of cases there is less than 1/10,000th of their population. (Even if one is skeptical about China's official count, it's preposterous to think they've undercounted by tens or hundreds of millions.)