Problems with that plan (assuming DO somehow gets enough resources to build a fleet for Humanity): A) Fuel economy, most ships can barely make it to the Reef and back. Some can get to Pluto. Good luck getting into another star system without risking using alien technology. B) Territories: Where lies the boundaries where Vex have molded space time into a single plane? Where lies the Cabal Empire (have you seen the cannons on the Imperial Land Tank? Actually, have you considered that that's what they consider a tank?!). C) Light, Guardians need it, At the edge of the system there is none. Oh, and the Hive are indeed on a Crusade to end all that is a source and is conducive to Light and even non-Guardians are the latter and they have inter-dimensional travel and weapons we don't understand.