I think you're confusing ecosystem (which word itself implies process) with equilibrium. It's quite fair to say that there was an actual ecosystem at least until the establishment of writing, at which point state information became transmissible across time and space beyond the capacity of individual organisms or eusocial colonies.

While you are correct that evolution would sooner or later take over, whether with assistance via cloning or blind luck if humans were to go extinct, you do need to consider the possibility that under some circumstances systems can collapse rather than bouncing back over a relatively short timeframe. Life would eventually re-establish itself, just as life persists in extremely arid desert conditions, but there's no guarantee it will do si n a timely fashion, and by timely here I mean on a timescale short enough for a human civilization to flourish in parallel with it.