The forerunners developed weapons with the same idea in mind. Take a look at forerunner weapons in Halo 4, many disintegrate their opponents, and were designed to do so so that the flood could not reanimate the fallen. And yet, the forerunners could not stop them. Because when you have a planet's worth of combat forms rushing you, your weapon's effectiveness against organics becomes secondary to how many you can kill before you're overwhelmed