People retire and lean on government services for healthcare and supplemental income - money that has to come from somewhere.

These systems rely on there being more workers paying taxes than retirees, and can’t survive when there are more old people than young people.

There are also broader geopolitical concerns - working age people form the core of your nations economic and military might. As that core shrinks, your nation becomes weaker in both a literal and economic sense.

For an island nation with a literally and economically hostile authoritarian power right across the pond, this is a grave concern.