That's a grave and serious insult to real literature, which obviously wouldn't sell like Harry Potter, Twilight or 50 Shades of Grey because most people either can't understand the writing or don't have the patience for it. Obviously, I've never read Harry Potter or most popular fiction because my time is precious and I devote it to only serious literature. I think the word you were looking for is 'fiction'. I'm not saying the series is bad (I wouldn't know, having never read it); it's just not serious literature, which isn't debatable. I mean, Rowling isn't a Barnes or a McEwan or a Rushdie. It's like comparing the ATP #100 to Federer. They work in the same field, but aren't in the same league quality-wise. You wouldn't study Harry Potter in a Literature class; you would read it for fun, some readers find deeper meaning in it which is great because that's what books are supposed to do, but it's not literature. I just cannot get over how you're claiming that the series is some amazing literary masterpieces. It's seriously offensive. I'm sorry, I love literature more than tennis and even Federer, so this is just...
That "serious" thing it's very debatable. It depends on what you consider serious or not. In music for example there are musicians that write music much more complex or elaborate and people think the music it's not serious because the lyrics talk about fantasy or fairy-tales or whatever and if another musicians write a 3 chord song but the lyrics are a philosophical essay they go "Oh, wow, what a masterpiece".
My point is that it doesn't matter what you write about, but how you write it. I don't think that writing science fiction will instantly make it less worthy. Some of the greatest authors write science fiction (Wilbur Smith, Stephen King, etc) If you didn't read them as you said, I don't know how you can be so offended. Give them a try at least. You'll see they are great and their success is justified.
And you can't seriously compare it to twilight. Twilight books weren't that successful or praised as HP, never. The movies were very successful because they're full of cliches and they cast two good looking guys for girls to watch the movie.