Oryx was only kept away from the City by the bravery of The Awoken. They stopped his fleet, so that all we had to deal with was him and the Taken. I could easily see a Cabal fleet ("They blow up planets and moons simply for getting in their way...") rolling in and blasting the City. We missed the full power of The Cabal because they were focused on fighting the Vex and then the Hive when Oryx arrived. But when we took out their leadership in system..., Well, if you're going after a giant, you'd better kill it. Or else you'll just make it mad.