Transhumanists: a new term for me. I like it. The people of which you speak are, in my opinion, very anthropocentric. Singularities, artilects, giant machine civilizations, all seem to be artifacts of technically minded thinkers who find it difficult to imagine how civilizations could evolve in any other way than according to their own prejudices. You got it right: pencil-necked geeks, techno-groupies, Machine Heads (apologies to Deep Purple).

For any civilization to generate a machine sucessor, it must be too stupid to anticipate the danger this robot culture will present to itself and take steps to prevent it, but still smart enough to simultaneously create an eternally self-perpetuating machine civilization by accident.
I can see someone too stupid to control their own technology (as I suspect we are) or clever enough to put together an unstoppable one (as we may someday become). But I can't see any civilization being both at the same time. No singularity will be born capable of immediate and constant self-correction. It will require a long period of dependence on its biological creators to perfect its self-perpetuating properties. Any civilization that will help its robot pets achieve that independence without ensuring its dominance over them will probably lose its control and be destroyed before the machines are ready to solo.

You can't rule out anything in this universe, and no doubt singularities have occurred, and artilects exist, but I'll be willing to bet they are neither common, nor very long-lived. Most civilizations either die a natural death, perish by accident or stupidity, or settle down to a stable and productive long life maximizing whatever activity they feel is most valuable for them. No doubt machine civilizations will do exactly the same. Robots don't need to conquer or discover anything. Endless, ever-increasing industrial expansion, exploration and conquest are not necessarily a characteristic of all civilizations. It isn't even an unavoidable characteristic of human cultures. Both natural constraints and other civilizations tend to resist this pathology and keep it from taking over. At worst, it despoils one planet, and then sputters out, poisoned by its own toxins.