1. The forerunners were defeated by the flood because all their technology was being logic plauged. The flood were infecting reality which messed up forerunner slipspace. And MC only defeated prometheans. He didn't even kill the didact in halo 4 because he comes back in the comics. A single forerunner war sphinx has the power to take out cities. 2. The flood gravemind can live until there is no more biomass to consume. They can use Star roads which would be indestructible considering the destiny universe has no neural weapons such as the halo rings. The guardians considered the dreadnaughts main weapon to be threat but the flood can just use a star road from across the galaxy to destroy any planet they want withen minutes. 3. Granted I will say that the halo universe cannot kill oryx or any other hive gods but the forerunners or flood can possibly contain them.