I reject the idea that population must continue to expand to sustain a country, when common sense tells you that an ever increasing population places unsustainable demands on our resources as well as our ability to tolerate close captivity. Human beings can stand being jammed together, but not if they feel trapped. People can live in New York, because they can go elsewhere for a break. Those who don't go elsewhere don't seem to do as well.
I live on a Peninsula. In the last 15 years, the trip from here to the other side of Tampa (ie escape) has changed from about 40 minutes to an hour up to two hours. The 75 mile trip from here to Orlando has gone from an hour and a half to up to four hours. Our experience with population growth is that the new people, be they immigrants or native Americans, will live within 50 miles of the coasts. That land is full. The only place to go is up, and that sucks.
I believe that you can scale down a country to work with a smaller population. In fact, it's happened in the past. To the horror of ten cent millionaires that means that a young person can actually save for a house or farm, because the price isn't rising 10% per year. Our wealth is somewhat artificial, and the only way for some to maintain their wealth is like Wayne Huizenga, to bulldoze and build on the Everglades for ever more people. Perhaps that's not the best way of doing things anymore?