So what stops 0/5 Mordekaiser from ulting the enemy carry out of the fight for 7 seconds and making a massive impact regardless of poor play? Will there be any risk to losing the 1v1? How about stealing Morde's stats if he loses? Something.
His ult will likely end the moment he loses the 1v1 inside of it. If a 0/5 morde ults the enemy carry I doubt he will last a whole 7 seconds.
What'd stop him is him dying.
He could stall it for a time, sure, especially if he grabbed both Zhonya's and GA...But if that ever became an abuse case Riot could just make them exclusive to each other since he'd be the only champion doing that. And he'd still die anyways, potentially letting the enemy carry come out ahead in health if they have enough sustain.
Also toplane is typically the one with the tankiest champion. Remove the tanky champion, and the rest of his team will probably just get wiped by a Kat passing out on their keyboard.
