A lot of those empty cities (well, districts actually) don't have jobs anyways, so no one actually wants to live in them. In fact, the only reliable way the Chinese government has in getting people to live in them is to move government offices to the area (so civil servants have to live there at least), and then there are problems as the speculators have already come in. This is kind of what the government is trying to avoid in Xiong'an by disallowing speculators (good luck with that).

At this point, there is no way for the housing market to crash gracefully. Maybe in 2011, but not in 2018. The amount of leverage offered up by Chinese banks this time means that most speculators can't simply wait out a downturn.