American capitalists and economic planners fret about "Japan Syndrome". To have more productivity and more consumption i.e. GDP growth, you need more people as a core driver. We don't actually need this, we could do fine with a stable population, but capitalism needs to grow or perish.

Declining populations are trickier for most economic concepts though. Less labor, less consumption. That said, a slight decline can leave more houses unoccupied which can be good. A major decline would mean so many unoccupied houses that you would have broken and abandoned houses though because it would be too costly to deal with the abandoned units.