I'm not sure I agree with your hypothetical archeologist's assessment. We haven't seen any kind of real drop in the amount of garbage being produced[1] (it's barely leveled off in the last decade, and that doesn't include commercial waste). Sure, the kind of garbage is changing, but that happened in the past as well (from bronze garbage to iron garbage), and it indicated a change in technology, not a failure of the civilization. And an indicated change in technology is the pointer our future archeologist will need to start looking for something else.

1. http://www.epa.gov/epawaste/nonhaz/municipal/