Earlier quoted context omitted.
Japan has 126 million people, while in 1910 it had 50 million. Populations are at similar historic, unprecedented heights throughout most of the world, developed and developing. We are nowhere near having to worry about extinction due to not enough people.
Please read the PDF I linked. The whole point is that even massive populations can be subject to permanent declines resulting in extinction under the right social conditions. One might as well say the two thousand rats in the article had nothing to worry about because there were so many of them - but the peak is exactly where the social collapse was irreversible. Again, I agree that seven billion is not a sustainable…
Not to mention, rat society doesn't change much whether it's ten rats or a hundred. Human culture becomes very different as people aggregate together, so whose to say that the very same social conditions triggered by increasing population will not themselves be changed by decreasing population?