"The idea that our primary worry should be that maybe in 300 years countries will be unable to handle this simple technical task that the US figured out in 1960

"The idea that our primary worry should be that maybe in 300 years countries will be unable to handle this simple technical task that the US figured out in 1960s is bananas." I suggest you open a history book. It is quite likely the same countries that exist today do not exist in the future and the same knowledge does not either. Furthermore it is not necessarily about being unable to understand some physics and processes but instead that language and knowledge quickly "deteriorates" over time, e.g. the langauge today is vastly different than even 100 years ago - technical terms and the like shift the fastest, cultural ones the slowest, but they do all become obscured by time. Even a more advanced civilization may encounter challenges that have no logical correlation with the task of keeping spent fuel safe, e.g. intential security measures such as encryption/obfuscation/code rot/reliance on no-longer manufactured tools. Anyways there is another objection you are fundamentally missing is that the timeframes are possibly un-sustainable in terms of time per acres. Sure you have storage for 100 years in dry casks (maybe some parking lot) but then must wait 100000 years. So you must store 1000 more times the fuel for it to be truly sustainable, digging giant holes in the ground is def not sustainable. Maybe parking lots are, but then you need to be sure your mountain of nuclear waste is stable (harder to do).