Magneto exists to remind everyone of the ever present danger of genocide. It's not something relegated to ancient history or WWII. Google the term and you'll see valid reports of genocidal practices being carried out around the world today.
I'm not quite sure why they never bothered to make Erik immortal after the concept of Externals was introduced. All it would have taken is a simple retconning of Erik's origin. Something to the effect that he actually died at Auschwitz as a child, to be resurrected in the mass grave as an immortal. I think a story like that would only add to Erik's trauma and his legend. It would also bring an end to all of the various contrivances and plot twists designed to de-age him.