Seriously, your complaint about the Harry Potter films were that they needed another dozen of them? By the time they finished filming the last one, Harry, Ron and Hermoine would be 30 years old!
Your mistake is saying that Hollywood not giving what the "hardcore fans wanted" was a problem. Those movies didn't make billions because of hardcore fans, they made billions because of their broad appeal. Make people have to watch two a year for 10 years and remember all sorts of backstories and plot twists, and they WOULD only be watched by the hardcore fans. And they'd end up making the same amount of revenue they made from the 8 films with 20 of them - but have 2.5x more expenses.
Hollywood is a business, always has been. Turning books into movies that even the most hardcore fans won't find fault with is not a way to increase profit, it is a way to create movies that are impenetrable for the casual moviegoer that get rejected at the box office.