Hi rexgorilla! Honestly, it's interesting to see this debate played out. I myself, back in my adolescent phase, engaged in a similar debate with some people on this thread that got nowhere. I meet people who are just as adamantly convinced about the inherent unsustainability of cities, and people who make proposals about their feasibility. I'm not an expert, so I can't know either way, and I fought some people regarding the debate with research that wasn't really perfectly accurate, so after getting emotionally involved in it, I finally realized it wasn't worth my time to debate it.
Personally, I've seen so many different people with different debates regarding what constitutes a "town" and "city" that I have concluded that at this point, "the jury is still out" regarding what permaculture can support, given that it is a new term defining many techniques that are found not just by indigenous people, but also by small-scare farmers in agricultural societies as well.