They’re not all actually decreasing in population, they are increasing via mass migration, legal and illegal. That is how the elite strata of Western European societies and the Anglo sphere countries (US, Canada, Australia) are dealing with this.

In Japan, Korea, China, Hungary, etc they are indeed decreasing.

I wouldn’t necessarily say that a society where the marriage rates are so low is stable though. For some definition of stability yes, but usually we also mean healthy when we say stable.