Many countries have 'eminent domain', with varying systems of compensation, usually property is inherited. Evicted villagers displaced by the three gorges dam, complained their compensation was negligible. Deng changing the laws in 2004, to support ownership of private property was a surprise.

The rent period you describe seems very shady

A land grant contract ... entered into between the land user and the land administration department of the people’s government at municipal or county level... obtained... by agreement, tender or auction

and since

there is no unified official procedure for the rules on the registration of land-use rights and the ownership of property. All interests in land must be recorded in the official government register. This register is proof of ownership

seems liable to all sorts of horrible abuses.

First it would seem to require families to rerent their own properties every 70 years. This might be legitimate if they called it a property tax, but instead just seems like a shakedown.

Second it seems if your family has property, someone richer or better connected to the party could tender more privately or outbid them at auction and buy it outright, or simply get them evicted and be gifted it by the government. It almost goes without saying foreigners need not apply.

Your remark is confusing though. Under this sort of system, the poor or politically isolated seem at the mercy of the wealthy and well connected (who have all the motive to stay and continue reaping benefits, none to leave). There is a saying 'possession is nine tenths of the law', and leaving China would seem to make anyone very vulnerable to losing their property, influence and wealth.

It doesn't explain the flight of the wealthy, unless they are taking a lot of currency with them, and aren't concerned about their property in china. That seems to agree with this.