Flip covered part of what I had planned to write (about the origins of division of labor/specialization/objectification of other life). Another piece, as I see it is purely practical. Where will we get what we need to maintain massive urban populations? Food, energy, and all the infrastructure to maintain the flow of these things.
Here is the normal answer I would expect to get: "Cities don't have to be what they are now. Without capitalism, people would be free to create urban farms, redesign transportation systems, and so on. The power could come from wind and solar energy, or in some places wave-driven energy."
Granted, all this is true. But then there are more questions - where does the water to irrigate the crops come from? Some of it in some places comes from gathering rainwater, but definitely not everywhere (imagine Los Angeles trying to maintain civilization minus the state without the massive diversion of the Colorado River, or perhaps the residents of Dubai trying to be more than a small coastal village without imported water). We import it then? What do the people upstream and downstream, or from whatever aquifer is having their water sent to wherever think about that?
Speaking of shipping, another issue is what fuels transportation (and industry)? Solar and wind power require both the raw materials (highly toxic ones in the case of solar panels) to harness them, and they also require an energy system that relies upon extractive industry (mining in this case). Who is mining those? We can recycle and repurpose some, but who is working in the highly toxic repurposing centers? How is this different than what we live in now? I suppose it could be entirely mechanized, but that just takes us back in the flow chart: where do the robots come from (raw materials, energy, etc) who builds and maintains them?
Oh, the waste. Where does it go? Civilizations create waste on a massive scale, and while grey and black water systems can work on certain scales, they don't address the toxic waste of civilizations. Where I live the salmon are having a harder and harder time coming back to their home streams not only because of dams and toxins and everything else from civilization, but because of coffee and curry. What I mean by that is the concentrations of these things (and other spices) that leak into the sea that they travel through via urine, groundwater leaching, etc is inhibiting their sense of smell, which, it turns out is a big part of how they navigate. Back to what Flip said about not valuing humans over animals, this is a real life example of the impact we civilized humans have on other species that isn't even addressing things like animal agriculture.
Most of the answers I hear to these lines of inquiry are that people will willingly choose to do that work, or that it will be worked out free of capitalism. The thing is, civilization has existed without capitalism. It wasn't any better.
While a lot of my perspective on all this is informed by anarcho-primitivism, I hesitate to call myself such. I think it is entirely possible people can create a freer reality using some pieces of what exists now (I wouldn't say I am hopeful about that on any sort of large scale), and I don't think a wholesale return to the Pleistocene is likely either. I just don't seen any of these things as compatible with the world I would like to live in.