You're completely right that given fertility rates are under replacement rate—from memory: 2.1 children per woman—populations will decrease with mathematical certainty. But there is no universal law that developed countries must remain below replacement fertility levels; that's just what has always happened so far—given the cultures and policies which exist today.
It's not completely absurd to imagine scenarios like:
- a rich society getting serious about the fertility problem and putting it's money where it's mouth is: free childcare, free fertility clinics, generous subsidies for having a third kid, etc
- a culture shift amongst the patriotic, or the religious, or whatever subgroup, to believe it's their responsibility to have more children.