In terms of reporting, I believe that you would like to see this in the Statistics page.
Because the statistics page is just counts of things, you’re wrong about where I (and probably others) would like to see this. Inactive user reporting is most useful for managing those users (notifying/disabling/deleting specific users). For that reason it would seem to me that it would be better implemented as a search criteria in the System Console → Users section. Personally I’d be happy with a REST call that could give me a list of all users inactive for N days or more, though.