hmmmm, so the society now heavily relies on the robots to build and maintain it's city and tools - lacking a lot of knowledge as they've become comfortable in the self-replicating and sustaining layers? Similar to how excessive AI-reliance in modern day can lead to a "dulling" effect on creativity or thought processes...
It only makes sense that self-replicating robots could produce an ecosystem when the rules tightly keeping them in line start to slip, much like how society flourishes when humans begin to specialise in certain things. Once groups of people specialise in certain roles like farming, others don't have to worry about growing food and can become builders, painters, writers, etc - and I guess something similar happens here when many of the robots no longer have the sole function of maintaining the city.