70year is just a number. The urban land in China is state owned. It only can be leased, so it must have a number. If the building is still safe to live, the lease will be continued. I know this apartment complex in Shanghai. I have a relative used to live there back in 1990's. The apartments are small and very basic (1-2 rooms with shared bathroom and kitchen) because at that time, it satisfied the needs. All the buildings are about 4-5 floors. In 1950's it was in suburban far away from city centre. It was for workers. Today it is already in the urban area so the land values more. Shanghai city may build high rises to house more people and also offer better and bigger apartment for the existing residents.