Guardians are notorious for seeking out power and fame. If there were still spartans to kill and armor to haphazardly glue together then they certainly wouldn't become bored and commit suicide. The UNSC would not stagnate, your statement about spartan IV's are false. In this battle we have to assume that all forces and factions would work together against the other game/universe. So ONI driving apart the treaties (which almost certainly wouldn't happen by itself) can not happen. Also all factions are involved so we must include the city, the reef, the fallen, the hive, the vex, the cabal, the warminds, the nine, the covenant, the ex-covenant, the rest of the UNSC, the forerunners, the flood/precursors, as well as the spartans, ONI, the guardians, and the vanguard like you mentioned.