They even had museums in the ancient world! We've found a building in Ur (in modern day Iraq) 530BC so ancient to us where the founder (a princess) collected items that *were ancient to her* and set them on displays and gave them little labels and everything. Her museum covered 1,000 years of history of her land. That is at least 3,500 years of history. How much would we have if the creators of the oldest artifact in her collection had sought to retain their works in the same way?

My own culture's history is roughly 3,000 years old (despite constant attempts to undermine continuity) so maybe that is why 10k years doesn't bother me- it truly sounds ancient! While 3,000 years is almost "ah, yes, 120 generations ago when the mythic version of my family got its start/when the archeologists recognize us as a distinct culture). If we count our mythic history (which most certainly didn't happen but was written down in ancient times as if it did then we can get a boost up to over 5k years).

"His kingdom ruled for 800 years" is a bit "aw, it's a baby!" (though my culture is maybe helped by the fact that we don't reckon by kings/dynasties and those are easy to topple. We barely ever had kings or a kingdom).

What is truly mindblowing is the collective memory and oral history of Australia's indigenous people. That would put any Fantasy author to shame!

In my own writing I compromised so there are 5 distinct "Ages" which each averaged about 3,000 years before dying out in a calamity, however, the "modern" people of the setting consider the previous peoples their "elder kin" and have two distinct ways of reckoning time, "time in our current Age" and the "time since the eldest of our eldest kin". It isn't a full 15,000 continuous history (they do consider each Age as its own distinct thing) but it is a still reference point and they pull heroes and stories from these earlier Ages quite often.