Why didn't Yoda then just team up with Obi-Wan to fight Sidious? Given how close he came to defeating Sidious on his own that seems a logical decision and next step. Did they think that Yoda barely escaping and Anakin surviving suggested that the Force wasn't with them at the time? That Vader or the Empire overall would provide Sidious with too much security for them to attack? Or that Yoda and Obi-Wan were most valuable as teachers to Luke and it would be too dangerous to risk losing one or both of them before they taught him?