It's really hard to say what the carrying capacity is, because unlike other species, humans keep modifying their environment. Without clothing, the answer is p

It's really hard to say what the carrying capacity is, because unlike other species, humans keep modifying their environment. Without clothing, the answer is probably a few million. Without agriculture, the number is south of half a billion. With mechanized agriculture, large-scale land use changes, and artificial fertilizer, we can probably still double our current population. But at the cost of stressing the rest of the environment. I'd guess the "safe" level is somewhere in the low billions, i.e. 1-5 billion. I was mainly talking about the SF case, where lower global population should make it more affordable rather than causing things to be abandoned. This is what is playing out in Tokyo, which is still growing while the countryside is abandoned. As for the small towns, it sucks, but dying small towns has been a thing for quite a while now as agriculture has become more mechanized. Detroit's kind of the odd man out being a depopulating metropolis, so I'd actually argue that it's a terrible example, at least for the next 100 years.