I still don't buy that explanation that firing that one ring destroyed all the flood.
For example, there's still flood spores all over the place. There's flood spores on every ring as far as we know. And the gravemind could have infected many planets after it had gained control of the ship and could travel at will.
Yes, it would have destroyed the gravemind. But as they've already established, the gravemind can rebuild himself anywhere the flood have infected enough people. As established by the gravemind rebuilding himself on the ark.
Heck, the gravemind is smart, it would have dropped a few flood pods on the surface of earth before going through the portal, it certainly had plenty of flood pods that it dropped before the control room. One flood pod would be enough.
As such, Bungie never really established exactly what kills off the flood. If it's simply the death of the gravemind that does it, they should somehow have established earlier that the gravemind is a singular entity and that when he dies so goes the rest of the flood.
However, there's still the glaring plot hole of why the gravemind didn't die off when the all the rings were first fired, thus destroying the flood.
If firing the ring only stopped the flood from spreading again, but didn't kill all of them, then they should have established that at the end too.
It really looks like that for all their high budget to produce this game, they forgot to hire a decent writer. Because really, this is just sloppy writing that just a few sentences of dialogue could have fixed up. For example, Cortana could have gotten some info from the gravemind while they were linked up about how even his power has limits. If he's seperated from the main flood mass after having formed into a concentration, if he then dies with no flood mass nearby to transfer to, the whole the flood goes with him.
And for that matter, where did the flood come from? What were the sins of the forerrunners? Etc.. the list of plot holes and goes on.
The gameplay is great, and good gameplay can carry a bad story, and Halo's story, unless in novel format written by competant writers, is pretty poor on its own.