I don't understand what this means. They don't mind when poor people move out? Of course they don't. Unless you are going to legally stop people from moving

I don't understand what this means. They don't mind when poor people move out? Of course they don't. Unless you are going to legally stop people from moving out, you are going to figure out how much you can keep the factor you apparently need -- rich people -- while letting as many poor people as possible benefit from being in a mixed-income neighborhood.