The scope of what I'm talking about is bigger than yours. Cities are more efficient, and we need to move more people into them (not just the ones that are big right now, just to cities in general) to give ourselves a better shot at keeping our planet habitable in the long run.

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.

>There won't be equivalent investment in services, recreation, leisure facilities to cope, so they'll degrade.

That's what happens in suburbia. You build a lot of infrastructure for very few people. That infrastructure was partly funded by private companies or federal government incentives. Once you have to collect property taxes and maintain the infrastructure you've overburdened yourself and can no longer meet your obligations precisely because there are not enough people living in that area.