I have nearly a dozen nephews and nieces, they live the length and breadth of England. And they were all the prime age (around 7-12 at the time of the first book). All literate and intelligent, and well-read.
That should have put them at the epicentre of the HP explosion.
As it is, I think one of them barely finished one HP novel while the others failed to get much further than a chapter or two.
I am not doubting the power of the HP experience. But it was not universal for that age group. Perhaps my family has a natural genetic immunity; but I never had to buy HP-themed presents for an entire generation.
Perhaps the HP-apathetics are a genuine, silent majority, that are worthy of some academic study.