Why shouldn't demographics be stationary?

When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes

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).

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An emphasis on higher quality, longer lasting durable goods would help, as would the recapturing of the productivity gains into labor rather than into the investing class.  This would enable us to do at least as well with less effort, and that effort could be spread over more people.

Why shouldn't a car last 20 yrs, and you not have to be continually buying one?  Why should a factory, like the one I used to work in, that needed 4,000 people and now needs only 1500 to produce the same output, still have 4,000 people working less hours yet receiving good benefits.  We shall have less, we had best learn to make better use of what we have-it can easily be sufficient.

"I said, 'Wait a minute, Chester, You know I'm a peaceful man...'" Robbie Robertson