High Population Density Triggers Cultural Explosions(esciencenews.com)
esciencenews.com
High Population Density Triggers Cultural Explosions
http://esciencenews.com/articles/2009/06/04/high.population.density.triggers.cultural.explosions
http://esciencenews.com/articles/2009/06/04/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.