Two things.

The first is that that is actually crazy late to me. Asbestos has been in use since antiquity. I am genuinely surprised that something so toxic wasn’t noticed earlier. Then again, in times where tuberculosis was common I suppose it wouldn’t have looked that odd.

The second is that you’re viewing it through a modern lens, where of course literally everyone should believe and know that it’s bad the very first time someone notices it. The reality is that it would be much more murky. I would not be at all surprised if microplastics are viewed the same way in 100 years; how could they not have immediately known it was bad? Because we need to quantify how bad, and we can’t just force feed it to people so we have to wait until we naturally get case studies.