User blog comment:Damous/Why Bioware has NOT betrayed us/@comment-4922072-20120313045408/@comment-68.7.49.118-20120313162320

With the destroy option the Citadel getting destroyed would mean that there is no Mass Effect field surrounding it anymore. That means that it would be impossible for Shepard to take a breath if he was still on the Citadel in that final scene.