The Chinese Communist Party can offer their population two reasonable claims to legitimize their rule: 1) we're responsible for this economic growth 2) we've made China militarily strong and nobody can push us around anymore.
If economic growth stalls long term that leaves only the military strength argument, and for that they need convincing enemies, and for enemies they need conflict. The ghost cities are an embarrassing failure, there will be an irresistible temptation to change direction and attempt to kill two birds with one stone, to reinforce both arguments for legitimacy by spending recklessly on the military to stimulate the economy and muscle their way into new resources.
Obviously this path greatly increases the likelihood of war with the United States.