To have a global negative population growth rate could only happen in the case of a severe epidemic, something like the Spanish Flu did a 100 years ago (~100s of million died then).

This would not really be good for anyone.

This presupposes that humanity is better off with fewer humans. I am far from convinced that this is the case.

Sure, if you want society to collapse.

Are you suggesting we should annihilate the human race altogether?

Uncontrolled loss of reproductive ability could easily spiral and be the end of our species, that is assuming that we haven’t had the same affect on other species, which is not a given either.

“Children of Men” outlines what that might look like.