High Population Density Triggers Cultural Explosions

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.