I went to see the sequel to Stephen King's 'the shining' last night instead and thoroughly enjoyed it. Also quite dark and definitely not aimed at children but oh the nostalgia of being younger, watching the shining through my fingers in fascination and horror.
Nostalgia marketing is huge business, its the adults that spend the money, and we're all tied to things from our younger years. It's how Harry Potter manages to churn over millions of revenue each year still, adults who read them as teens or children have children of their own, and the obsession gets passed down. They've remade ghostbusters, jurassic world game out, they even remade jumanji. Doesn't matter that a lot of the remakes or sequels aren't that good, people will want to see them because that was a part of their younger life.
They don't aim these remakes or continuations of childrens/YA things FOR younger people, not really. Stranger things is full of 80s references and homages to classic films, songs and games from the 80s,it goes totally over children's heads but we remember all that stuff... It's for the now-adults who enjoyed these things. If kids enjoy it too, then bonus.
I won't be watching HDM purely because I didn't like the books or the film!