> I stopped seeing caps in gutters.

Do you think that people who were inclined to discard bottle caps were correctly binning their bottles?

No, they were also throwing down their bottles which were more easily carried away by the rain and wind. Now, both the cap and the bottle are being washed into rivers instead of the caps being left behind. Net improvement: zero.

It's a classic example of survivorship bias. It's like the headline that bottle caps were the third most common litter on beaches - because bottles, sandwiches wrappers and crisp packets had been washed out to sea.