Doom only has one trick; to steal power from others while speaking in the third person. He has taken over the world and failed at the follow through. In that sense, he’s a paper tiger. Like Osborn, you can just wait him out. Magneto isn’t out to rule the world or get ahold of a cosmic cube. He knows his enemy and has the ability to face them as-is. He wouldn’t plot to take down Doom. But the latter has prepared to fight Magneto. Why would the boogeyman need to do that? Again, the X-men have moved closer to Magneto ‘s way of thinking. These were the individuals responsible for his past defeats. This was one of his goals; being in a position to steer mutant kind. He has what he wants. Doom just got pants-d on tv.