All models are wrong, of course, but this is kind of "common sense" so it's not hard to accept as true in a natural system. How can something continue on exponential growth forever without reaching a new blocker that causes slowdown or encountering pushback that makes it an oscillator. A pendulum looks exponential when it is at its peak and accelerating down.

The issue is that the exponential-looking part of the sigmoid might contain all of human history, sure, but most folks who espouse this theory probably agree that over time everything reaches a steady-enough state to be considered non-exponential, or become oscillatory.