This fight was the culmination of his long years of training, the climax of the Schism. The Sangheili hadn’t expected such a vast and well-prepared mutiny, and in the moments before the human frigate infested with the Flood slipped into the dome, the Prelates and their Jiralhanae crews were winning. But then, one by one, the Prelate-controlled warships had peeled away from the battle to evacuate High Charity’s San’Shyuum. What had been a perfectly executed surprise attack became a defensive scramble as the Prelates switched from trying to defeat the Sangheili warships to merely keeping them at bay while the San’Shyuum filled their own ships and slipped away. At first, the Sangheili let these vessels go. Then, as the threat of the Flood spreading beyond High Charity increased—as the Flood spilled down from the dome to the stalk where the rescue vessels had been docking—the Sangheili sent a message in the clear: ALL SHIPS ATTEMPTING TO LEAVE THIS SECTOR WILL BE DESTROYED.

Truth knew only a blessed few could follow him on the Great Journey. And after the Sangheili committed the ultimate sin of losing the first Halo ring, they quickly fell out of favor. So the Prelates redoubled their clandestine preparations and, as much as Rtas hated to admit it, had the Flood not intervened in the battle for High Charity, the Prelates likely would have succeeded in carrying out Truth’s wishes.

-Halo: Shadow of Intent

If the flood doesn't take over High Charity, the brute and prophet controlled ships wouldn't have had to focus on evacuating High Charity and would have won against the elites. If Truth ended up getting the reinforcements from the High Charity defense fleet, he would pretty much have been uncontested in travelling to the ark and firing the Halos