[twoz]

High population density leads to greater exchange of ideas and skills and prevents the loss of new innovations.

Loss of life and innovations can still happen with high density population centers, although less often (as long as information is also exchanged/shared between centers).

Higher population density also means more expensive and more deadly natural disasters.

If, today, a devastating earthquake/hurricane/volcano eruption/mack truck sized meteor hits NYC there would be enormous consequences.

The same or worse event occurring 100,000 years ago wouldn't have been such a big deal with no people around.

I think the future is very bright yet any sort of disaster your can think of will be leveraged against us that much more by increased population density.