Assuming negative population growth ends in a total world population of zero is as insane as thinking positive population growth ends in people stacked ten deep in Antarctica.
It seems that way and of course it likely won’t get that extreme but the scenarios aren’t symmetric.
Extinction is a real phenomenon. There have been many extinctions is your life time. It is directly a physical possibility both from a logical standpoint and an evidence based standpoint point. Like this isn’t just math it’s reality.
People stacked 10 deep is not a physically possible scenario. There aren’t enough resources on earth to sustain that level of population.
The maths works out, but the reality doesn’t.