Because he might just be the only champion who relies 100% on this particular item to succeed.
It's his only core item and while Morde builds may differ they all have this one thing in common.
Having abysmally low base MS, health and resistances makes him extremely susceptible to kiting and getting his three Q's off is what he relies on in order to succeed.
Tampering with this item will totally turn his balance upside down and seeing that he's in a solid spot right now makes me kinda scared of what will happen to him once preseason hits.
So if you're going to make it less desirable on him please give him the Illaoi treatment and mini-rework him to work without rylais.
But if it's going to be shifted into less damage and more utility like you mentioned everything's going to be fine and dandy.
I just hate how one item holds him hostage in his current state, though I really like the slows on a thematic standpoint (He's like The King of the Wild Hunt or Arthas with his affiliation to slows and the cold grip of death)
Anyways, I just don't want Mordekaiser to suffer, he's been through enough.