My answer is, maybe. You're argument admittedly leans on pure speculation. Could the flood win? Yes. Could they not win? Yes. A problem I see with your logic is that you treat the Republic like they are the Forerunners. They aren't too similar. Now throw the Empire into the equation and the whole scenario can be changed. Instead of a blockade, the Empire may just destroy the planet and everyone on it. Therefor the flood goes with it. The Empire would do whatever it takes to kill the flood. Just like how they reacted to the Yuzong Vong. Which the Yuzong Vong would also be on their side too. That is just a recipe for no survivors.