You never know if they are completely dead. Like it is said they both practiced resurrection. But since they are also Hive Nokris may possibly have a throne world like Crota and Oyrx. So we may see them again either in a raid, or in the next expansion ressurected. Does anyone remember how many damn times omnigul was resurrected? The Dark Below, Taken King, and Rise of Iron.
LOL yeah. Nobody really thinks about the lore and tries to apply it to the game itself. they just go oh these are extrememly powerful beings so they must be hard to beat. They don't take anything into account about the setting of the game.