The question is how effective that justice system was, not whether it existed or not. It's the same with medicine, it existed for quite some time but the effect

The question is how effective that justice system was, not whether it existed or not. It's the same with medicine, it existed for quite some time but the effectiveness increased drastically and you wouldn't want to roll back all that progress. The reality is that for most of those tens of thousands of years justice systems were mostly a sham where the focal point wasn't actually determining innocence but rather the appearance of justice and pleasing (some) people.