Is that wither really necessary?

{quoted}

But a slow that increases 95% though? On such a low CD? I don't mind him having a slow, but 95% seems a bit much for anyone

It's only 95% for the last second at most, and due to how slows work (you can't go below 110 movement speed) the final 95% is around as useful as the 50%. Otherwise, it's his only CC (and a soft CC at that) on a heavily kitable melee who has the majority of his damage in melee range. He needs that slow to even try and function.