If anything, this might strengthen the argument by giving us reason to think at least some ecosystems have additional new protections that can reduce harm from acid rain.

This indicates that certain things that protect against acid (acid-neutralizing buffers) have decreased, which would tend to increase harm from acid rain. This increase might offset the effect of decreased acidic pollutants.

This suggests that the decrease in acidic pollutants will end, which implies that we should not expect harm from acid rain to decrease in the near future. Once pollutant levels stop decreasing, we would expect harm from acid rain not to decrease.

This suggests that decreased amounts of pollutants in acid rain don’t significantly impact the harm caused by acid rain. Thus, we have less reason to think that decreased acidic pollutants would lead to less harm.

This suggests that protections against acid rain have decreased, which would tend to increase harm from acid rain. This increased harm might offset the benefit of decreased acidic pollutants.