There are many unscientific ideas that somehow manage to accumulate followers, and sometimes political power. Look at astrology, socialism, global warming, etc. No end of examples. As I read it, Asimov says that creationism is an idea that has lost out in the scientific arena, yet continues to gather followers from elsewhere in society.
If the proportion of congenital idiots born is invariant, then the the absolute number of such idiots must rise with the increase in the world's population. Of course that increase in total population is totally due to scientific achievements in producing and distributing clean water, healthy food, and medical "miracles" to the world, so maybe it's just payback.
Perhaps that's the point he's trying to make.
However, I think there is a growing percentage of scientists who say evolution is not the answer, whether or not they embrace creationism or intelligent design.
I for one think that when you look at all the "assumptions" that an evolutionist makes. That there is room for any number of alternative ideas.
It continues to astound me that in an age where man is capable of genetic modification and producing designer crops and animals, that evolutionists are closed minded to the possiblity that man himself may have been engineered.