Earlier quoted context omitted.
I am not convinced that it is a solved problem. How old are Egyptian pyramids? Yet there is a lot of stuff that we don't know about them. We are confident, that after 20 000 years, people will still know about nuclear waste storage facilities? Think about it: records could be lost, our language will be different. Why are we so sure that somebody will not dig it up?
The radioactivity of spent fuel decays exponentially. After a few hundred years, most of it is gone, although there is of course the danger due to the chemical toxicity, just like any other heavy metal deposit. Obsessing over what could happen if somebody digs it up after 10000 years is not anchored in reality.
Meanwhile, "According to a recent model study the total anthropogenic mercury released into the ocean is estimated to be around 80,000 to 45,000 metric tons" [1]. We have spewed so much mercury that people are advised to eat less tuna because they have too much mercury.
And mercury is not radioactive. In other words, 24,000 years later, there will be 100% of the same mercury there is now! (Granted, some of them might have been removed from the ecosystem and deposited deep underground... I mean, just like how plutonium is stored right now.)
[1] https://en.wikipedia.org/wiki/Mercury_pollution_in_the_ocean