There is no reason to compare single mechanics. Champions have a certain power budget, and while most people agree that Kalista is over-tuned, that does not change anything. They wanted kalista to be a high mobility, good utility, and lower damage carry. They also decided to make her damage very back loaded. As opposed front loaded damage like graves, or consistent damage like Vayne.
When they look at what needs to be done with Kalista, first they need to decide if the mobility is just too much and needs nerfed, or if the damage needs nerfed to compensate for allowing the mobility. If they determine that her damage needs nerfed, then they need to decide if they want to take away from her sustained damage, or if they want to take away from the back loaded damage of rend.
If they decided to take the damage from rend, then there is less of an incentive to save the stacks and Kalista might start using the rend primarily for the slow and she would still have her sustained damage. So maybe instead they should leave rend very strong and just nerf her numbers. That way Kalista would still have to make the decision between the slow and damage at times.
Twitch has absolutely nothing to do with the process of deciding what to do with Kalista. Who cares if one champion has a similar mechanic on an ability. And who cares if one of the abilities is clearly way stronger. It does not even matter which champion is stronger overall. Why limit yourself to designing a champion because of another champion? As I already said, Twitch has way more damage overall, it's just that twitches damage is not focused on contaminate. Would buffing contaminate but nerfing Twitch's sustained damage (leaving Twitch not buffed or nerfed) change anything about the discussion of Kalista?