It's entirely possible that the ability of doctors to diagnose even severe autism has improved, for example a family doctor who sees a young child with very low IQ may not be surprised by undeveloped language skills and may not recognize that as autism specifically. It's also possible that the increasing destigmatizing of mental/developmental disorders and increased awareness of autism in general could result in more parents and doctors seeking proper assessments. The sad reality is that there are still places where autism is likely to be seen a spiritual condition, a witch's curse, or demonic possession.

Well we used to just let those people die. You know how the medieval family had like 15 children and 4 survived to adulthood? Some of those 11 were severely autistic.

That would be relevant if we compared to the 10th century. We're comparing to the 1960s and 1980s.

>but not the case

like all things in life, it's not the only variable. but better recognition is absolutely a factor.

What is the incidence per million over the last couple of decades?

I have twin cousins who have severe autism, one of which is institutionalized. When they were very young we (their caregivers, family, and doctors) understood their diagnosis to be "mental retardation." Nowadays they are known as autistic. They didn't change, but the world around them changed so their diagnosis shifted.

Before there were government services families still had children with profound disabilities that required lifelong care, they just didn't usually have any formal diagnosis. Everyone still understood that they need help with everyday activities and can't live on their own but sending them to specialist for a diagnosis didn't seem like something that was needed or even available if desired. The diagnosis is needed if you're applying for services, so more services that exist means more formal diagnosis will happen.