I think the answer to your question is that most people impacted to the severe degree you're imagining don't live till 70.

I don't know what the condition was, but I had an aunt who was affected by 'something' and was disabled in many of the same ways you imagine; she was nonverbal and needed full time care from a young age. She lived to 43, and towards the end they had to put in a feeding tube for her.

I have a couple of questions for you:

1. Have you ever personally seen a 'severe case' of autism where someone needs full time care for their entire life? Where are you getting your information about the degree and prevalence of this?

2. Putting "cases are clearly on the rise" alongside "where are the 70 year old severely autistic people" implies that you (via RFK Jr.) think that the number of severe cases is on the rise, not just that more people are being diagnosed with some level of autism. Do you know severe cases are on the rise, or are you (or RFK Jr.) just making some assumptions?

The reason I ask #2 is that the 'severe autism' we're imagining in this narrative would be, as you point out, obvious. But what if our understanding of symptoms has gotten better, and we're diagnosing more not-obvious cases, i.e. not severe cases.

Please keep in mind that the first person officially diagnosed with autism only died a couple of years ago. Donald Triplett. He lived to be 89, by the way.