It is important to keep in mind that actual autistic people's traits and experiences are not as clearcut as these levels present. Many autistic people have a wide mix of traits associated with multiple levels, so these classifications are not helpful outside of medical contexts. Labelling an autistic person as one level socially and seeing them solely through the lens of the DSM can lead to neglect, hindered potential, and harmful misunderstandings. (my underlining)
And there is the rub. I have a successful adult Ds with a seriously spiky profile. High IQ and some high abilities, low EQ and many low abilities, and "how much support the autistic person needs in daily life" is far more than might be expected, and most is just left to me to provide, or watch him sink.
Mismanagement results in disaster and can result in hospitalization with catatonia and long recovery periods.
All his energy has gone into managing work, and his ability to do anything else as well, is very limited. He hasn't reached anywhere near independent living at an age to be expected for his intelligence level, or relationships etc.
His life is limited to managing work which leaves a level of exhaustion that isn't anywhere near normal. Every few weeks he has to wfh or take time off. The first warning is waking up with crimson eyes from subconjunctival hemorrhages, total exhaustion and unable to prevent himself stimming. (he can normally suppress to accepted types and levels) By then his gait is also changed and he's struggling on many levels. By this time he's dangerous and his physical appearance is so changed that managers are concerned and send him home on H&S grounds. He's highly valued at work for his abilities, and they've accepted the downsides.
So should he nowadays be categorized as having 'mild' autism? Superficially possibly yes, but that denies him the protection and assistance (not financial) to operate needed for him to contribute to society without extreme risk to himself, because using the level system, he looks like he should fit level 1, but fluctuates between 1 & 2 with some dips into 3. Thankfully he has an older diagnostic label of 'profound Aspergers' that includes 'multiple exceptionality' and a recognition of a spiky development that's still ongoing, so is more protected legally once he has included his documentation at interview.
There is also a relative in Broadmoor who could pass as a 1. Until you discover the 'reasoning' process behind killing his mother and his failure to understand or deal with change. "how much support the autistic person needs in daily life" was very misunderstood, and left to his mum.
On the other hand there is a family friend, now adult, non verbal, in nappies, aggression is their only communication, and needs everything done for them at all times. They fit neatly and clearly into 3 as 'severe' in a way that NT folk can grasp, but even then "how much support the autistic person needs in daily life" is acknowledged, but still often left to his mum.