I wouldn't call Guardians a superhero movie either. There's no costumes, no secret identities, no real superpowers, and the only codename is mocked in the first scene. It's simply a space adventure movie.
Easily - sort of. You know the movies would capture a fraction of the BO that the main book movies had. At a certain point you're catering to the long term fans rather than trying to get as big an audience as possible. And that can work when you have a limited budget - franchise sequels are the most safe investment of any kind of movie - 85% of them deliver at least $300 million in box office. Still, if you're a studio with spots for only 3 franchise sequels a year, you'd need to be in a place where the the source material has run dry.
I mean the LOTR movies are good for a billion each probably, and New Zealand will do anything to get something new going, but I haven't heard of any rush for a Silmarillion trilogy. Would it really pay for itself?