I don't think there's anything inherently wrong with taking a sparsely plotted novel (like The Hobbit or Foundation, both of which have a lot of details happen

I don't think there's anything inherently wrong with taking a sparsely plotted novel (like The Hobbit or Foundation, both of which have a lot of details happen off-stage and sometimes bafflingly so) and fleshing out the details. The problem with the Hobbit movies is just that they did that very badly.