Why does this argument always come up for autism specifically?
It is TOTALLY NORMAL for a diagnosis or a medical condition to encompass people with a huge range of different experiences with that diagnosis or condition.
Put ten randomly-selected people with a diagnosis of multiple sclerosis or dyslexia or Ehlers-Danlos syndrome in a room, and you'll have ten very different experiences. There will be those who could very well go through life without colleagues or acquaintances ever realising they have a condition, and those whose condition could never conceivably go unnoticed.
With autism and Asperger's, while the two diagnoses were previously treated as separate things because they could appear quite different, in reality they're very difficult to cleanly distinguish from one another for a big chunk of the people concerned. In the diagnostic manuals, the only real difference between the two conditions was whether there was speech delay.
When they looked at adults, they found that there were quite a lot who had had speech delay as children who were clinically indistinguishable from those who hadn't.
And there are so many families that have members with both Kanner-type (for want of a better word) and Asperger-type autism, as well as unaffected members, that it seems likely that underneath all these different types of experience within a family, there's something similar being passed on that comes out in different ways.
I would agree that there are probably several conditions that we currently diagnose as autism that actually have distinct underlying mechanisms and causes, and hopefully at some point enough research will be done to distinguish all of these and determine if anything should be done differently depending on the underlying reason, but there doesn't at the moment seem to be a good medical reason to split autism back out into separate conditions along the lines of Asperger's syndrome and autism.
People just don't argue this for other conditions as regularly as they do for autism — that people with the same condition or diagnosis as their children, but whose difficulties are less obvious, should be given a different diagnosis. It's considered quite normal in most conditions that the same diagnosis can apply to people whose conditions affect them very differently.
Is it backlash against vocal adult autistic (or sometimes dubiously self-diagnosed) online advocates and activists?