I'm a frog of little brain so I'm not sure this topic isn't way over my head. However, I agree with both of you.

Depression to identify sadness has little to do with depression to signify "I can't cope."

As to the reason depression is so over diagnosed....
Is it an umbrella type tag, a scapegoat diagnosis for "I don't want to research further".
Or perhaps it is the by product of the over stressed world we live in without the coping skills necessary to survive.

I do not have the answer.