Evolution is not like a conscious process that develops new traits in a species in response to a changing environment. When the environment changes, evolution can only work with traits that are already in the gene pool, all of which were created somewhat at random in the past (the random part being mutations). Autism must have existed in the past (I am not sure if you are saying that it didn't). People like Da Vinci, Thomas Jefferson, Ben Franklin (he would have been an unusually socially skilled autistic), Nikola Tesla, Edison, and even some people like Patton were very likely autistic. Heck, maybe even the first stone tool was invented by someone with something similar to autism.
The increase in the prevalence of autism may be a change in the way people diagnose it. Or if there is an increase, it may be for entirely different reasons. For instance, in the middle ages people thought that they should beat ret*ds because you think faster when you are scared. They literally thought that one could have sense beaten into one's self. I can hardly imagine someone with autistic behavior, even if they would be able to take care of themselves if left alone, being left alone in this sort of barbaric society. Maybe the increase in the prevalence of autism has more to do with people being less inclined to beat them to death or banish them completely from social life. Also, an increase in prevalence would have to be directly correlated to an increase in reproductive success of autistic individuals (otherwise a drop in mortality rate wouldn't cause a boost in the population longer than one generation). As far as I can tell, autistics are not renowned for their reproductive prowess. I am inclined to think that the increase in prevalence, if any, is caused by the general population no longer being cruel enough to keep noticeably affected autistics from reproducing at all.
But maybe you're right. I'm just not convinced.