The problem here is you are asking for the impossible: strong evidence that novel chemicals are safe. How could you ever get that? It's impossible to prove a negative.
And doesn't this just privilege tradition? Chemicals will be grandfathered in for spurious reasons, not because they are actually any safer. Famously, chemicals in plants very often light up the Ames test for carcinogenicity and would be ruled out by your argument if they weren't "natural".
We have to compare potential downsides though, we have two choices, we can:
* not ingest novel chemicals
or
* ingest novel chemicals that may be carcinogenic, but scientific research will not be able to prove anything either way
Do you not think that it is rational to choose option 1, given our understanding of the Lindy effect?
We have plenty of strong evidence for the safety of tons of things. My ancestors have been consuming cow's milk, mache, and wine for thousands of years. If these things were not safe for consumption, we wouldn't be consuming them to this day. My bloodline wouldn't have made it this far. We don't add poison hemlock to our mache salads because thousands of years ago, some poor souls gave us strong evidence that it's not something you should eat, and that knowledge was passed down to us.
And doesn't this just privilege tradition? Chemicals will be grandfathered in for spurious reasons, not because they are actually any safer. Famously, chemicals in plants very often light up the Ames test for carcinogenicity and would be ruled out by your argument if they weren't "natural".