To put that in perspective, let's say I drink water contaminated with chemicals for decades. Then, "suddenly", me and half the people i know are sick with cancer and various side effects decades later...

That's how environmental toxins work. They accumulate throughout the water cycle and through the food web, and if its less than acute (short term) in effect it statistically hurts a population, such as lowering reproduction or creating birth defects that lower the fitness. Then, once concentrations pass LD thresholds (lethal dose, meaning LD50 will kill half of the individuals of a species on average, LD10 would kill 1 in 10) you start getting mass die offs

Every water table, all of the soil, every living being is riddled with non-naturally occurring substances. Even though we released more damaging toxins in the 80s, the rate of pollution doesn't matter - the concentration in various parts of the ecosystem is what matters, and that's a slow process