When you say, “It depends whether you think severity means how far from typically developing one is, or how impacted one is by their differences. The impact of autism can be enormous and just as significant in an intellectually able, verbal person,” it comes across as pretty dismissive of what severely or profoundly autistic people and their families actually deal with. For people in that situation, they could have every possible support and accommodation, which would certainly improve their quality of life, yet they would still be very severely impacted in almost every part of daily life. Not being able to care for yourself and being reliant on others for every single part of your life, is being severely impacted, how could it not be?
Have you not seen that for yourself in your work with autistic people?
I agree with you that autistic people are “more likely to be misunderstood, isolated, lonely, overwhelmed, sensitive, and to struggle with perspective-taking, as well as being unemployed.” But all of that is amplified a hundredfold for someone who is severely autistic, non verbal, and often has an intellectual disability. They will find it incredibly difficult or impossible to make themselves understood, to express when they are frightened, overwhelmed, or confused. They will likely be unable to make friends or romantic connections, and they will almost certainly be unable to work, no matter how many accommodations are offered.
No matter how difficult they find social interaction, they cannot escape it because someone has to be with them constantly to care for them. They don’t have the option to arrange their life in a way that suits them, even if they had the best care in the world. Their lives are inevitably very hard, and there are ways to really improve their quality of life, but nothing that can prevent them from being profoundly disabled.
An able, verbal autistic person can in many cases, if they are well accommodated, find a life that suits them and where their differences are less disabling, or not disabling at all for some. Many will find work that suits them, date, live by themselves, have children, a social life. I did myself, as did many other people I know. But the severely autistic people I used to support could never do that. Severe autism involves huge challenges over and above those faced by people who have the privilege of being intellectually able and verbal.
And of course there are many people somewhere in between the two extremes.