That makes good sense. ASD is a catch all term for those with certain symptoms that are unexplained by other factors. Schizophrenia and anxiety disorders would be some of those other factors. Not sure about ADHD.
There is substantial comorbidity with ASD and ADHD. ADHD occurs in ~4.4% of the general population, but with ASD that goes up to ~45% (the margin of error is massive on the latter)
Some of the symptoms of ADHD, like RSD (rejection sensitive dysphoria), sound very much like ASD traits rather than ADHD traits.
In the future I think we'll find a lot more underlying commonalities of all of these conditions. OCD, GID (gender identity disorder), misophonia, tourettes and more are also strikingly comorbid within ASD.
The point that stuck with me is that, as you say, the high comorbidity is strong evidence that we have a bad model of the situation. Part of that for me is the medicalization of deviance.
As an example, I saw a bit on Twitter from an autism researcher who was sitting in on a young girl getting tested for autism. She was shown an animation of triangles moving around in a way that neurotypicals would anthropomorphize: https://twitter.com/DrRubySapphire/status/123278780736346112...
She described how the triangles moved instead of humanizing them. She recognized that they looked like they were playing, but knew that triangles didn't have minds. That was marked as a failure, a sign of sickness. Not because she was wrong, but because she was different from the norm.
I think a lot about the world where we just accept that there are a bunch of different kinds of people and stop trying to fit them all into one mold. Where we make the world work for all of them, not just the "normal" ones (which, historically, seems to mean straight, white, moneyed, neurotypical men of a particular body type).
They cast doubt on the biological validity of the distinction between ASD, ADHD and OCD.
Similarly, in this editorial – https://www.ncbi.nlm.nih.gov/pubmed/28714261 – Waterhouse et al write "The ASD diagnosis lacks boundary construct validity because 96% of those with ASD have significant non-diagnostic symptoms, including many who have full comorbid disorders"
