Thor exclusively defending Earth, whether that was Stan Lee's original intent or not, is boring. That's every superhero movie. They've spent two movies setting him up as a protector of both worlds, and we're already getting Thor based on Earth in the Avengers movies. Spending time on Asgard is what separates the Thor movies from the other movies, and it's what the general public associates with the character now. Besides, who are the "iconic" Earth-based villains? Absorbing Man? Already bastardized into the 03 Hulk movie. The Wrecking Crew? lol
The impending destruction of all of Asgard, the death of Odin, the unleashing of a feared ancient monster from Asgardian myth, the Ragnarok, Loki's "redemption" at long last, there's a Thor 3 in there. A good one. Thor flies to Earth, punches Mr. Hyde in the face, and poses for pictures? Not really.
Thor's budding relationship with Jane, Jane's visit to Asgard, Odin's disapproval of Thor's love for Earth and Jane, Loki's punishment and weaseling out of it, Malekith kidnapping Jane and betraying his soldier, Kurse's resurrection and lust for revenge, and Kurse realizing he was betrayed and getting his peace. That's dense. It's an entire movie. It's not the throwaway set-up for a third act doomsday scenario.