You need to re-read the Books of Sorrow then. You say that if the Hive don't hunt us, the Darkness will. The Hive are one of the "Darkest," enemies out there. Also, the Vex are a thriving civilization, who can actually exploit their time traveling abilities, but haven't yet. The Cabal obviously aren't just as powerful as we have seen them on Mars or the Dreadnaught. They have an emperor. Dead Orbit actually knows this, and shared the information with the Tower. Also, you say that we don't know if we'll find life-sustainable planets. There is a loophole in this. We have garnments that can synthesize a suitable atmosphere, as seen on every single planet we've been on with no air/safe atmosphere. Also, if Dead Orbit finally got their way and sent out colony ships to different planets, and found a planet with only element (minerals, fuel, water, etc.) resources, we could send Frames or Exos to obtain the resources. Supporting this argument even more, all organic alien enemies we face such as Fallen, Cabal, and Hive, prove that there are life-sustaining planets out in the galaxy. There are so many stars/solar systems in our galaxy that for every grain of sand on earth there are 10,000 stars. There is bound to be at least a few planets that can sustain life. PS: The enemies we face besides the Vex and maybe the hive have to come either from the Milky Way, the Andromeda, or one of the many dwarf galaxies that are in a cluster of celestial bodies called the Local Group. This is because the Universe is expanding, and it is expanding at an exponential rate, meaning it gets faster and faster as time goes on. When it expands, the bodies in the existing Universe have to fill in the space, and spread apart. The Local Group is held together, and not torn apart because they are close enough to affect each other with gravity. But, the other galaxies outside of the Local Group are moving away faster and faster, meaning that unless we find a way to travel faster than light, or we find out how to teleport, we can never escape the Local Group. Doesn't that make you feel lonely? Anyway, the Vex and Hive may come from different galaxies because both of them can teleport in some way.