> Barring a major collapse, we are unlikely to forget about them ...

It is not a given that generations that live through changing times (whether regressing/progressing technologically) will maintain the same priorities and convictions.

For example, imagine there are 100 (some number much bigger than 1) radioactive waste sites scattered around locations with different economic capabilities. Then a discovery is made that, though costly enables complete recycling/cleanup of the nuclear waste. Some locations are able to cleanup and some are not. The emergence of a solution reduces the perceived danger posed by nuclear waste. The longer it takes for all the locations address the issue, the easier it becomes to forget about it.