You're right that conclusive research can be impractical or neglected for many environmental and social impact "sciences". This is why psychology, ecology, biology, medicine and even chemistry weren't always taken seriously by more rigorous disciplines. No innovation really marked a moment between when they were denigrated as soft sciences that couldn't make sound conclusions and their role in society today. It's mostly just a cultural shift and a loss of implicit skepticism in them. So yeah, it's hard to know stuff like "how dangerous are plastics" as rigorously as "how much energy is released when these two nuclei fuse"

But there's also a practical weight that matters in a lot of these pursuits. Plastics overtook the world during a time when people weren't really considering their impact at ecological scale and didn't have sufficient models and tools to really assess what they might do to biological systems over lifetimes or generations. It was a historical accident that happened fast and without many brakes applied and now "plastics everywhere" is a pervasive background noise that's very hard to meaningfully control against. So the controlled science which was never conducted initially, and we now think probably should have been, is almost impossible to perform because we're left in that that fuzzy "soft sciences" place where everything is already plasticked.

But does that mean we should just keep charging ahead without trying to look, and that maybe a mindful step back from unscientific historical use would be worth considering even without conclusive evidence (since such evidence is now too hard to gather)?