I was thinking about an analogy using the "all in your head" term, but decided against it.
Technically, most things are "all in my head." I mean, if I didn't have a head, then nothing would bother me. It's where the brain is, and thus where everything is identified, it's also where thoughts of what to do and how to react are formed.
There is "mind over matter." But to even suggest that one can, with a wave of a hand, immediately change whatever is resulting in mental unwellness is ludicrous. It's a process, just like any illness is. Some take longer to heal, some only have limited healing.
What I stated before is that everything is medical..or rather physically based. That's how enmeshed we humans are made. There is no separation of mind/body... I personally think of it as a hologram in each cell: each identifying aspect and controlling chemical of the person is contained in each cell to some extent. My argument is that IF one were to say there is no "mental illness" it is only to use another term that fits more intrinsically with the medical terminology, with the physical illness genre (that happens to result in psychological distress also.)

But continuing to use the term mental illness is still quite acceptable to many people. (I just don't like the stigma that goes with it.)
We are fearfully, and wonderfully made.