Our whole economy and social system are designed for a growing economy, and a growing population. Without future growth, savings and investment become more necessary, but less attractive. Without growth, people become less generous towards strangers and more unhappy about their own circumstances. And without the growth around which all of our modern welfare states have been structured, the modern safety nets that governments have spent the last century establishing may not be politically or economically sustainable.
If you think that population decline is going to be a net boon to society, take a long hard look at Greece. That's what a country looks like when it becomes inevitable that the future will be poorer than the past: social breakdown, political breakdown, economic catastrophe.
Waronwomen! Waronwomen! There. Problem solved. --ts