I see that the population grows as the life expanse grows. In 1800, the health was lower, and there were less medicine than there is today. This is why people died more often. Now, we have far more cures, and we can heal people easily, compared to 1800.
The only way for the population to decrease is to have a massive disaster that we cannot avoid, and we do not know a way to solve it, either is is unknown, or we are too lazy to.
If a massive contagious disease comes, and we do not know how to cure it, we all may die, and the population will decrease dramatically.