I feel like people aren't giving the Fallen enough credit. They're survivors. They've threatened the city multiple times and can build advanced technology from nothing but scrap (i.e. Servitors, skiffs, ketches) We have no idea how many cabal are outside of our solar system. It's very possible the cabal would become extinct in OUR solar system, but how many live outside of it? There could be forces in other galaxies, taking other planets and we wouldn't know. The hive are reborn after every death unless they are killed in the ascendant realm. Even then, thousands are being newly spawned every day. They certainly won't be going extinct anytime soon. If I remember correctly, the Taken are like the hive and are reborn unless killed in their own realm. Time travel is not understood enough in the Destiny universe to tell what will happen to the Vex. I myself believe in time paradoxes, so it could be very possible to kill off the Vex if time paradoxes existed within Destiny. The different timelines may disprove time paradoxes in Destiny, however. If they truly don't exist in the game then there is hardly a way for us to totally terminate the Vex