How does that even happen, by the way? Humans survived but somehow lost all knowledge/language and all artifacts they could have used to bootstrap? I've never really understood a realistic sequence of events that leads to that.
I do agree though, that if they could do this in the Dark Ages, we've left considerably more artifacts around to do it with today. Any moderately large town has a library with enough information to get things going.
As we have seen in the last couple years, just a couple poorly timed pandemics can set us back 50 years or so. Add a meteor impact or a nuclear war and we are in for major chaotic transformation whose results can't be easily predicted.
On what metric? I can't think of anything. Medical outcomes, crime, wealth, none of that stuff has regressed nearly that far. Some social issues might have regressed to the early 90s if you take a pessimistic view of the situation.
If either of those happens, radioactive waste will be the least of anyone's worries.
Now consider how fast it would rebound given that those remaining 800 million would rebound, given that they'd have vastly more knowledge and resources (even just having access to pre-mined materials alone is a massive boost!).
It doesn't need to be humanity, I guess. It took humans what? 4 million years roughly to diverge from something like the great apes of today to anatomically modern humanity. Does nuclear waste stay dangerous for that long?
Sorry, I don't think there's any point in spending any time designing anything for that scenario (except as art or philosophy, but nothing practical). I feel like people are underestimating how resilient and embedded and redundant our society is at this point, and how very specific the scenario would have to be to lose everything yet humanity survives.
Huge asteroid impact could kill us all, but it doesn’t seem obvious how it could cause a reset to zero.