Mordekaiser can never be a true late game carry
http://www.surrenderat20.net/2015/11/red-post-collection-illaoi-introduction.html#2
The reason is simple: unless his numbers are so high that he 1 shots everyone (at the same time), a melee with no gapcloser or CC can never be a true scaling threat that long range champs or even a melee with gapclosers can be. Late game he's just going to get popped by a Vayne or Jinx before he can even react.
Why balance him as a marksman substitute when he won't ever be one without insane numbers?