So many people would not have died or been enslaved or been persecuted at the hands of true believers.
The believers would still exist. They would still consider themselves "true believers". They would just be wrong in thinking that what they believed actually exists.
You later said this:
No, god would be replaced with more rational explanations.
Why would "god" as an explanation be replaced?
Maybe you've taken the question to be where people all
know that god doesn't exist? Rather than god simply not existing, and people's belief in god being mistaken?
As a philosophical matter, it actually addresses an important epistemological point: do believers really
know that god exists. They may claim to. But what do they
actually know, and what of that would
actually change if god does not exist. Maybe god does not exist, and thus what they think they know is, however convincing, not true.
The same is true for those that claim to
know that god does not exist, so don't think this is an anti-theist argument.
I agree that if everyone suddenly
knew that god existed, or they all knew that god did not exist (and by "know" let's say that it can be demonstrated) then people would act differently. But the question was not that, but simply if god did not exist. And to think that people would have acted differently in the past, it in the future is to assume that people would somehow "know" that god does not exist.
Ultimately, either god exists or does not exist. If god exists, the world is as it is, and history is as it was. If god does not, and has never existed, well, the world is as it is, and history as it was. People still believe one way or the other, still convince themselves they "know", and none of what they rely on for that "knowledge" (or what they consider to be knowledge) would be any different.