Earlier quoted context omitted.
Funny because cities are more efficient from an energy and carbon footprint perspective. The only thing that makes it unsustainable is that some cities haven't figured out how to make it so everyone who should abbe able to live there can afford it
They're already more efficient though, and there's no benefit to the current residents (tech rich or not) from massive incoming population growth. There won't be equivalent investment in services, recreation, leisure facilities to cope, so they'll degrade.
To your second point - degradation of infrastructure is not an issues with cities, it's an issue with management and investment. On a practical level, with additional people should come additional industry and tax revenue which should in turn allow metropolitan areas to keep pace infra-wise.