Some people think we will either poison ourselves like bacteria in a closed environment or that a plague will come along, killing all but those naturally immune ( which is also seen as an inbuilt mechanism so the species survive)
In the past, plagues decimated populations, keeping numbers down.
Now we have medicines, but the viruses and bacteria are fighting back, themselves becoming immune.
So in a way nature is looking to right the balance.
Antibiotics are becoming less effective, there are superbugs in our hospitals.
We are in a constant war with nature to keep the balance in our favour.
Some people think one day our luck will run out.