Water for one. It was very risky as things like droughts quickly killed you. It was also very risky as someone moving upstream of you and shitting could see you dying from dysentery very quickly. Water is in far worse shape now because of how deeply we've pumped out aquifers and how poor we've left soil conditions in many places.

Next is amount of people. Current human density is supported by antibiotics. Take away them and we quickly fall back to around 1900 population density (1.6 billion roughly). And not even internal antibiotics, external antibiotics like chlorine for cleaning and water purification.

So those are the setups for population collapse. When population starts collapsing this way it generally overshoots the numbers pruned because of war/disease. We won't fall to 1.6 billion, it's likely to fall well below 1 billion.