Star Wars has been generating tons of spinoff material for decades now with little signs of fatigue in the base even with the Prequel era. Only difference now is that stuff that would have been relegated to novels and comics in the past is being filmed or animated and that its more then just publishing now.
Guardians of the Galaxy can only go so far on smartass characters and 70s/80s Jams in space. Particularly when Fox still controls a ton of Marvel's cosmic stuff through the Fantastic 4 and X-Men rights. Only so much you can do when you're restricted from using Galactus, Silver Surfer, Skrulls, Shiar, the Brood, and Mojo. If they ever do get X-Men back, a Guardians movie with Mojo as a villain would be fantastic.
Guardians will be around for at least another decade. We get a movie next year , they will be big in the next avengers and then we should get another movie early next decade.
As for Guardians you can use the Nova corps and their baddies and also simply create new characters for the movies.