To retcon the entire Halo universe up until this point would probably be the worst narrative decision in the history of the franchise. They wouldn't be able to do it in a way that didn't feel cliche/contrived and it would be a giant slap in the face to every Halo fan.
343 has already retcon several moments from halo from them nerfing the actual power potential of the ur-didact in Halo 4 to high charity surviving the events of Halo 3 in Halo Wars 2. There's also the fact that Del Rio was supposed to be one of the UNSC's brightest military captains which is why he was chosen to lead the UNSC Infinity, which we know is blatantly false (as scene by how he "lead" his men in Halo 4). There's also Cortana surviving the events of Halo 4 (surviving rampancy and getting nuked) and becoming the villain in Halo 5 for reasons that still aren't very clear,. There's the fact that the Didact was somehow defeated by a Pulse Grenade in Halo 4, coming back in Halo Escalations and immediately "dying" all over again by standing in a spot he could have easily moved away from while Chief super composes him. There's also the fact that 343 Guilty Spark somehow survived the events of Halo 3 as revealed in the novel Halo Primordium even after being destroyed on Installation 04-b which was subsequently self-destructed thereafter). Halo Reach's story also cannot exist while the events of the novel, Halo: Fall of Reach, are still canon.
The universe needs to be rebooted to some extent, but nuking the entire expanded universe would be a terrible idea. Some things need to be cut out and others need to be changed (Halo 4's story and Halo Reach's story, should be redone to fit the expanded lore, but the basic story arc and noble team and how they each die should remain entirely untouched).