Sure, it might be popular with schoolchildren...for a time. But typically, they would outgrow whatever series they were reading and move onto other things. And where the hell did this crazy adult fanbase come from?

Have you missed like the last century or something?

Comic-con, star trek conventions, anime conventions, etc. Star Wars weddings. Star Trek weddings. Marvel weddings. These are dominated by adults who are spending shit loads of money on their fictional worlds and associated products®. Harry Potter is just one of many, many fandoms.

One old parallel -- Civil War reenactors. No "consoom product" but people who are obsessed with a story, events, etc.