I think you are extrapolating a little too much from your personal experience. An obvious counterexample to our society trending to "shorter and shorter forms of engagement" is the rise of long-form podcasts and long-form independent reporting.
Plenty of people still enjoy literature and the way in which it can convey ideas that non-fiction does not. My impression is that you'd have the same reaction to more recent fiction by, say, Cormac McCarthy or David Foster Wallace. I'm not sure Harry Potter is exactly a fair comparison of a work that aims for the same register as Moby Dick. As far as I know, 19th century literature with a simpler prose style (for example Sherlock Holmes or Poe's short stories) is still entirely accessible to someone with basic reading comprehension.
> Call me Ishmael. Some years ago—never mind how long precisely—having little or no money in my purse, and nothing particular to interest me on shore, I thought I would sail about a little and see the watery part of the world. It is a way I have of driving off the spleen and regulating the circulation. Whenever I find myself growing grim about the mouth; whenever it is a damp, drizzly November in my soul; whenever I find myself involuntarily pausing before coffin warehouses, and bringing up the rear of every funeral I meet; and especially whenever my hypos get such an upper hand of me, that it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people’s hats off—then, I account it high time to get to sea as soon as I can. This is my substitute for pistol and ball. With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship. There is nothing surprising in this. If they but knew it, almost all men in their degree, some time or other, cherish very nearly the same feelings towards the ocean with me.
your brain will click right in.
Entire patterns of thought become antiquated as our communication wiring adapts to the ever increasing pace of society. Social media is driving dopamine hits from shorter and shorter forms of engagement.
It's amusing to think that Harry Potter might one day read like an opaque relic.