The developed world needs domestic population growth to continue. If you have fewer children you're handing the future off to other countries (either via immigration or competition via trade or military conquest) which do not share your views on sustainability or the environment. Some of your ideas here make sense but not having children does not.

You certainly save a great deal of energy in the short term for both you and the state. Long term this is extremely destructive. If you want to see how it ends look at former/current communist countries like Russia and China.