Okay. Idiocracy is a movie. A funny one, with something valid to say, but a movie. If the world got to the point where it even resembled that, civilization would have already completely collapsed or it would be subsisting on automation that the previous generations built. In either case, you have bigger problems than some radioactive waste leaking a little in Nevada.
Much of Nevada is a marginal place for humans to live to begin with. If there was a catastrophe that eliminated a lot of people, those people wouldn't go living in Nevada near the nuclear waste site. They'd move to the places it was easier to live. Just like before the Black Death in Europe, the development of marginal lands only continued profitably (or at all) while there was high population, and thus demand. Kill off a third of the population, and they stopped developing marginal areas and depopulated them.
The real risk of the waste site is increased expansion of human civilization which puts a lot of humans near the site. This isn't like Chernobyl or Fukushima where fire and explosions are spreading the material. We're talking more about material leaching into groundwater and things like that. A terrible thing to happen, to be sure, but not exactly a problem if no one is living there.
Compare this to your Roundup example, and it is apples and oranges. Herbicidal treatments will be applied to locations where weeds need to be killed for food production. That is a much more serious threat compared to some nuclear waste stored in casks under a salt dome in the middle of nowhere.