Well to me he looked like X men's deadliest foe. X men have beaten Magneto so many times, he doesn't look threatening anymore. I mean he doesn't really want to kill the x men, so that might be a reason.
Considering it took Wolverine's attempt to kill Magneto for Mags to finally decide to rip the adamantium from his sekelton shows he's exercised great restraint on his powers in their meetings. The limits of his power is unknown but he's shown great power in the past (bringing Kitty home in her speeding bullet across the universe for example). If Magneto were to run with an all-out-war mentality, it's tough to say that anyone could stop him outside of cosmics, as he can control the very planet.
Cable, sans techo-vi..., would be one of the most powerful mutants on the planet. Just look how powerful his clone Stryfe was. While not a mutant, Black Bolt is another powerful being capable of taking on Apocalypse.