It’s their nature anyway, by the standards of higher beings. The thing you might be missing is that it’s billions of different versions of stupid.
But getting back to what you were saying, there’s no realistic danger of all humans believing in nothing just because they don’t all believe in some particular thing.
What is the danger of people losing their faith in some particular thing? As this guy pointed out, humans are hardwired, to some extent, to engage in “religious” i.e. worshipful and ritualistic behavior. If they lose one religion, they get another – even when it’s something they think isn’t a religion.
Can the replacement be worse? Yes. But there will still be something. And people will often find their somethings compatible with other people’s somethings, which after a bit of compromise and revision results in a more cohesive something. Like breaking eggs and getting an omelette, or some other egg dish, or at least a realization that salmonella poisoning sucks and that broken eggs are a nuisance to clean up.
Truly believing in nothing (unless you mean the concept of Nothing™ – which is actually a variant of Something™) isn’t really possible anyway, unless you reduce the brain to the point where it’s barely recognizable as human.