Those explanations are not at all opposite. They are different factors contributing to the same observation.
There are fewer developmentally disabled children these days. Because many of the children having easily diagnosable ones (trisomy 13, 18, and 21) are aborted, there is a higher proportion of children having non-trisomy developmental disabilities.
Fewer children being institutionalized means there is a higher likelihood of seeing any child with a developmental disability. Since the proportion of autistic children is higher in that group, there is a higher chance that when you do see a developmentally disabled child, he/she is autistic.
Another factor that I forgot to mention before is the role of random distribution. Clusters can form purely by chance, so you may happen to live in a place where several families with autistic children randomly moved in. Also, is there a school for the developmentally disabled nearby?
There are many possibilities. There is no reason to assume that the incidence of genuine autism has increased, without accounting for all of the other factors.
I have similar discussions with people all the time. It’s either the weather, “global warming”, vaccines, and other issues science related.
You handle it well.
I see no reason to assume it hasn't, especially since the perception that autism has increased is not mine alone, but is widespread. There is no need to protect either hypothesis.