Facts have proved that the government's method of "blocking" the inflow of migrants into big cities by raising the threshold and the cost of living is not feasible. To fundamentally solve the problem of population congestion in big cities, the most important thing is not to block, but to sparse; not to keep human capital as the most important factor of productivity out of the door, but to guide the retired elderly people in big cities to rural areas which can improve their quality of life and the level of elderly care by means of opening up and market voluntary. Only by realizing the two-way flow of population in China's big cities, can the cities be healthier. "Replacing the elderly to make space for the youth" can not only control the population size, but also optimize the human resource structure. What's more, it can alleviate the increasingly serious urban diseases in metropolises, and finally, it can resolve the prominent contradictions caused by housing problem and high housing prices in big cities.
