But in a democratic frame work it is unlikely that you get majorities by telling the people you can come along with less even if you could have more.
And it is a question of speed of change as well. Currently in Germany about 50-60% of the children are born, which would be necessary to keep each generation the same size. 80-90% would still be environmental responsible and would be easier to handle from an economic point of view.
In an aging society feeding and clothing are not the only important issues. (Health) Care for the elderly e.g. is another thing and I don't believe that communism works. So you can not redistribute everything. As not all countries are going to follow, you would see a lot of talented hard working people leave Europe (e.g. to Switzerland).