The Vex have been seeking to convert entire planets into machines and write themselves into the fundamental forces that govern the universe. That goes beyond trying to simply survive. The Cabal are conquerors. They already have other worlds under their control. If they're seeking survival, it would be easier to fortify the worlds they have instead of spreading out their forces and taking distant planets. As for the Fallen and the Hive, just take a look at the mounds of human bones that litter their lairs. That's not survival, that's genocide.