What I would like a change in the diagnosis to accomplish is for it to better represent my child. I'd like to be able to he has x diagnosis and for there to be an automatic understanding of how his needs either present to other people or how they impact him similar to how saying autism prior to the mixing of dx did.
This might have been a nice perk of the old split system that you were able to take advantage of, but it's not one that diagnosis always does, or necessarily should, confer. I can see why you're frustrated to have lost such a useful shorthand, but it's not a convenience someone is automatically entitled to when they get a diagnosis of something. Lots of diagnoses cover a vast range of severity from barely-noticeable to completely incapacitating. Lots of diagnoses cover wildly varying presentations with completely different symptoms. ASD is no different.
Just to say my answer above was mainly in response to "confusion will ensue, and individuals will find that neither diagnosis really suits" as people are already confused and the diagnosis now doesn't suit children like my son. I guess it isn't perceived as the serious disability it is because many with the dx aren't as impacted or if they are it's dye to mental health issues.
Many parents have to give a short explanation of their child's particular difficulties and needs and can't expect a single word to do the job. The broad ASD diagnosis suits your child, just as the broad diagnosis of asthma suits the child with life-threatening brittle asthma. I'm sure it's frustrating for that parent of a severely asthmatic child if people think of asthma as just getting a bit wheezy sometimes, but that doesn't mean mild asthma isn't asthma, and I never see asthma parents campaigning for mild asthmatics to be split off into a separate category with another name. (I know I'll now likely get a response delineating all the ways asthma is different from ASD, which is really not the point, and ignoring everything else I've said, but such is MN.) When I talk about neither diagnosis suiting, I don't mean people who don't like that their diagnosis also covers other people who are different, I mean people who straddle the categories, and with autism you'd likely get that problem again if you re-split it.
Being able to quickly communicate about an individual is one of the purposes of a diagnosis, but it's not the only one, so it can't be the sole consideration. You could campaign for a split back into completely separate diagnoses depending on severity, on the basis that for some people, it would sometimes be a little more convenient, but your preference would have to be weighed against the medical, scientific and practical reasons that the categories were merged in the first place, the impact on all the millions of other people who have or would merit an ASD diagnosis, on science and research, on public confusion, on the logical consistency of the categorisation, and so on. IMO the current state of knowledge on ASD doesn't allow us to cleanly divide ASD the way you would like.
However, I would like to see an end to people jumping down the throats of those who try to find language to accurately communicate what the autistic people they care for have difficulty with. If people are made to feel they're not "allowed" to describe their child as having "severe" or "profound" autism, or to refer to them as non-verbal or low-functioning or having challenging behaviour, are not told any suitable or useful terminology they can use, and are simply told that all autistic people are autistic and that "mild describes how you experience my autism", then it must be tempting to wish you could go back to just being able to say "autistic" and it mean "like my kid".