How are these low numbers? Also Halo games have legs, with its amazing multiplayer it will continue to sell months on end. It will probably end around the 4-5m sold in my opinion.
As for the franchise's health, the game raked in 400m+ in its first week, i dont think game sales matter to MS anymore since they have already made there money on it. Halo 6 is only going to improve on this. Halo is far from dead.