Well, your point is well taken. And of course it would be possible to build economically stable societies with contracting populations. But probably not in the short run. A longer term shift in economies to bio-regionalism or localism, less dependence on global trade and supply chains, more local small scale agriculture and a value shift to less resource consumption, renewables, etc would be feasible and needs to happen. But in the short run with an aging population even that can become difficult as there is a lot of labor that simply can't be done as easily by people on the back side of middle age. Not to mention, not enough young people working to support retirees via social security becomes a problem too. So barring a shift to guaranteed income, and letting AI and robots do all the difficult labor, for the foreseeable future, declining populations create problems as is happening in Japan currently and has been happening in Italy for a while...