On the other hand, if the birth rate decline results in a significant population decline (i.e. not corrected by migration) then there are various balancing econ

On the other hand, if the birth rate decline results in a significant population decline (i.e. not corrected by migration) then there are various balancing economic factors that should counteract it on the scale of a generation or so. A major component in the affordability to raise a family is the cost housing in desirable locations, which is mostly driven by a population increase there, and would fall during a population decrease; and a decrease in work-aged people combined with an aging population obviously pushes salaries up, if other things in the job market stay equal. And if we're speaking about the world as a whole, we're so far from a population crisis - the current trends of global birth rates still imply a population growth until something like 10 billion; while even a halving of population from the current number to 4 billion wouldn't be something to be terrified, I mean, the world had 4 billion people during 1970s.