Outside of this example, proving a negative on this scale is a 1000x larger problem than proving there are no leprechauns. No one has proven there are no leprechauns. It is logically possible to prove a negative, but we simply cannot observe all of the places a leprechaun might be hiding at the same time. The best thing we can do is list off all the places we know there are no leprechauns. This is very different than "proof".
That's not proof. A ten year drug trial may fail to show problems that take 20 years to surface, but that doesn't mean the drug is safe.
I suspect a lot of these chemicals fall into that hard-to-find-problems category. Only when they are an overwhelming part of our environment for decades or even generations will we start to understand the effects.
So this is not just a matter of money. It's a hard problem.