“If they don’t change.”

Yes, it’s called math.

In any case, plenty of civilizations have been destroyed by collapsing populations. The precedent is pretty clear. And the math of replacing humans with robots is pretty clear as well: input and maintenance costs are far less economical for a lot of labor than relatively cheap humans.

So as the human population declines it becomes more and more expensive to upkeep the complex systems that allow for modern society. We can see this vulnerability today with the strain on the logistical system and supply chains and energy costs. A hypothetical future of robot workers will be exponentially more complex and exponentially more vulnerable to disruption.