It is not an inverse pyramid though. The older humans are the more likely they die. So you always and up with a pyramide at the top, at least somewhat. With low birth rates a society has to care for fewer children. That results in an actually fairly stable ratio of working age population to dependents.
A shrinking population also means build infrastructure is already built. They just have to keep things running.
People are always born with the same age namely 0, but they do not all die at the same age. In fact getting older increases chances of death. Hence 2 babies per mother ends up in a pyramid too.
Even if you presume people all die at the same age, things will be stable. If say people all get childten at 30 and only work between 30-60 and then all die at 90. If we then assume 1 child per couple and everybody has a child at 30, we would get a stable dependency ratio of 2.5 dependents per worker. Obviously those numbers are not realistic. Btw that also is not a pyramid, but a trapezoid.