There are still many things we don't know about the Gravemind, such as the way it moves around so quickly. Destroying High Charity only upset Gravemind and did little to slow it down, as can be seen on the final mission in Halo 3 where it was still talking to you as you are climbing up the structure to fire the ring. Maybe firing the ring did destroy the physical part of the Gravemind, but since it is an omniscient being, and since all Flood carry it's knowledge in some shape or form, it matters very little if its physical body was destroyed as a proto gravemind(s) are made while the active Gravemind is 'alive' so another Gravemind can be easily made. Even if for some reason there is no proto gravemind in the galaxy, there are still for sure Flood alive as the ring was fired outside of the galaxy. The Flood are the Big Bad along side the Precursors (assuming that some did actually leave the galaxy and their motives are evil and they do eventually come back) so they aren't going away. The firing of all the rings technically didn't stop the Gravemind, it only slowed it down for a hundred thousand years, so I doubt firing one ring will do anything.