What i get out of their mission is that dead orbit doesnt want to just take all the loyal gaurdians and bail on the city, they want everyone to come with them, civilians of the city, the vangaurd, everyone possible or willing. Which is essentially what they did, albeit rushed and temporarily, when they evacuated to titan. Which didn't turn out so great. They are probably rebuilding their fleet, scouting for a better, safer area(read: not a hive breeding ground) and gaining as many people to their cause as they can.