because the oral tradition survived and so did the written tradition, that was not the case with avestan.My bad, I misread your comment.
But aside from the fact that what you say about the Avestan is wrong, the Rig Veda has the problem that the oldest extant copies of the text date to the 1st millennium CE, not 1500 BCE or anytime around then. So how do we know that the text wasn't corrupted in the intervening 2500+ years? In fact, how do we even know the original composition date of the Rig Veda when the oldest surviving copies are thousands of years later? Isn't the idea the Rig Veda dates to ~1500 BCE and that it has remained relatively unchanged ever since originally a product of Western linguistics? Isn't Western linguistics wrong and racist and full of anti-Hindu/Indian biases?