His passive does assassinate people, the burst from it in conjunction with Q, E and W passive is quite formidable on full AP Ekko. His slows are enough to deliver all 3 in a time frame that doesn't feel completely unfair to the person hit.
If you remove the slows from his kit, he'll lose basically all forms of utility and force him to go Rylai's, Gauntlet or Gunblade, turning him into an AP tank at best.
Ekko's ult damage got nerfed because he was winning teamfights with it. If people can't dodge an ability with an 8 second delay, how do you think they'll feel about a faster (W) Q E R combo?