You have come up with an interesting argument. So in your opinion we cannot talk about global overpopulation, only about local overpopulation? And there is local overpopulation at some places, but there is no overpopulation at other places? I'm thinking on how to relate my concept of global overpopulation to that...
EDIT:
Well, my definition of global overpopulation would be a level of human population size that is dangerous to the environment, potentially causing mass extinctions and a big crisis that maybe destroys 50% of humans in about 5 years... the numbers are just my estimations, but I think you can get the idea.
Let me explain, some towns in Japan can do with more population since they were made for it and proved to be able to sustain it for centuries, they are underpopulated. Then you have the plains of Bihar, which have fertile lands and abundant water from the Ganga, but there is no way it can support a population density of 1400 per square kilometer, the consequence is the state has a reputation for all the wrong reasons and it along with 2-3 more states ends up ruining the stats for most of India. Then there is my state of Himachal Pradesh, boy, we are one of the happiest and richest among all the large states, our stats are slightly messed up due to the tribal regions in the north, but in their defense getting modern facilities to an altitude of 4km and -20degree celsius with the most inhospitable terrain there is, is rather difficult. In the Himalayas the environment is geographically highly sensitive, it cannot support too many roads, tunnels, hundreds of houses etc. and thats why things like landslides or casualties due to floods are increasing. There is nothing like "global overpopulation', it can just be readjusted.
A mass extinction for Humans in the foreseeable future is unlikely, but smaller ones may happen, here is an example,China is currently building the world's largest dam on the Brahmaputra river, which is bigger than the 3 gorges dam by a huge margin, but, this dam is in the most seismically unstable zone on the planet with nearly a billion people living in the downstream in India and Bangladesh, but since the dam will lie on China's border, they literally don't care, since it will have the highest output from a dam in the world, besides they can use it choke India's water supply in the north east o use it to unleash a mini Tsunami in India. That kind of thing might happen, but is that thing breaks, It will kill nearly 0.5-1 billion ppl, and then it will probably also start a nuclear war which no one will win. I recommend you do your own research into this.