Mordekaiser was a petty excuse for a metal/magnet champ to begin with.

One of the first things that comes to mind with a metal/magnet champ is CC. Mordekaiser had none. Instead, his entire kit was based around the idea of out-trading an opponent, killing him, and bringing his ghost back to life under your control. Mordekaiser's new identity makes more sense and fulfills that idea better. Now, with his kit reworked, and known as something else, Riot can make an actual metal/magnet champion with abilities that make sense with the theme.