It was mentioned about the Culture forging a peace between the IoM and the Necrons - it'd take full-on intervention with Culturetech, but it wouldn't be impossible. It's another way the Necrons don't fit the standard HS models, in that they don't actually care all that much about the people, they just want the real estate back and are perfectly happy to let the current residents go live somewhere else. Yeah, planets are small and weird and get hit with stuff, but they matter to Necrons.