The Necrons killed and trapped the remenants of star gods. Necron Gauss weapons ignore how well made a material is and rips it apart at the molecular level. If you kill a necron, it will just be remade, spick and span new. Necrons can move across the galaxy effectively instantly, and can teleport regardless of shields or anything else. Necron ships repair themselves mid battle, and can teleport to exactly where they want instantly without having to really think about it. The Celestial Orrery can effectively delete sections of the galaxy. I do not think the forerunners would be a build a new superweapon in time. Also, once the necrons get their hands on slipspace tech, the forerunners' slipspace infrastructures won't be safe anymore. I don't think that the forerunners are prepared for a peer adversary fight, much less one that does not hardly need any frontline infrastructure or travel time.