The principle of tradition and trust in the people of delivery

Hello Rabbi,
I wanted to ask, there is an argument that sounds not very reasonable to me, but it is also famous and well-known. And it is that people in the distant past were gullible and stupid. So it was possible to instill in them almost any historical idea, even if it happened by chance.
For example, during the time of King David, it was possible to instill in them a belief in the existence of the Ten Commandments and the status of Mount Sinai, even if it was a total forgery. Our suspicion of the people of the past consists of two main reasons. Their analysis of reality was completely wrong (relative to modern eyes). They thought that almost every phenomenon in nature was the work of the gods, and along the way they also exaggerated and immersed these interpretations in myths and legends. Second, they did not have enough documentation to remember the past. And the little documentation that we see that they had was full of mythical myths and urban legends.
Combining these assumptions, it follows that if an impressive person came to people in the past, he could sell them almost anything that was done to their ancestors in the past (say 300 years ago). Even about a seminal national event. So if nowadays we expected them to be supposed to ask why we hadn't heard about it. Once upon a time, they didn't have this minimal sense of criticism either.
What does the rabbi think about these things?
Sorry for the length, but I don't remember this question being asked on this site, even though it's asked so many times.
Regards
K.