I appreciate that Eliezer tries to explain the Death-Eater point of view - they're heroes in their own minds; and they're the only ones trying to solve a terrible problem that the "good guys" are ignoring. He also points out flaws in eg Dumbledore (though that may be dumbing down the character). Overall, his treatment of the conflict is more balanced and nuanced than Rowling's. More the kind of thing that I think I like (though I could be deceiving myself).

But if the book had been written that way, could it have been a bestseller? Is stupid moral oversimplification necessary in a mass-market bestseller? E.g., Tolkien, Narnia, Star Wars.

I'm trying to think of mass-appeal war stories with a balanced or ambiguous or at least non-stupid treatment of good/bad, but the ones I come up with are not exactly blockbusters: Gormenghast, Ender's Game, Grendel, The "Good War".

Some blockbuster movies qualify: Saving Private Ryan, High Noon, Blade Runner, Watchmen, The Searchers, Rashomon, Apocalypse Now, Unforgiven. Odd that movies, which are thought of by intellectuals as more lowbrow than books, may be more successful at communicating non-stupid ideas.