Everything can be an adaptation, even maladaptation can be an adaptation in general. The debate between adaptation and illness is not productive. One should ask why at first place people try to describe depression, e.g. the phenomenon of a sustainable wish to quit from life, by medical terms. The answer is that whatever the reasons and mechanisms involved, despite that the really depressive people by definition do not ask for help because they do not believe in it and because by definition they prefer to die soon, the non-depressive ones cannot stand depression around them and ask and pay their scientists to do something about it.
But what about depressive people that actually ask for help? These people suffer from dysphoria, not depression as such. Dysphoria is a symptom like fever or pain. It is a kind of pain actually but not focused on any body part. Many causes, external and internal, exist for fever pain and dysphoria. Many people in chronic fever, pain and dysphoria may at some point express a general wish to die but this is not depression. So why psychiatric science is accepting and perpetuating the confusion between dysphoria and depression? This is something one should answer in political terms.