Kalista is low winrate because of her damage, not because of her ult

Winrate in the high 30s/low 40s is pretty atrocious. Most people seem to be attributing this to her ult being difficult to use without some serious communication. However, after playing with her, I find the ult isn't really that hard to use. There are some really obvious use cases for it, and the support usually can figure out exactly what you're trying to do - either save them from being focused (and then possibly re-engage) or just flat-up engage on a foe. Once you know the dash range, it's not much different from using any other ult.

So, what is her problem? The reason why she can't carry games is because she doesn't put out enough damage. She has no steroid in her kit, so despite having great kiting power, her attacks don't hurt that much on their own, and her abilities, though they may appear to at first glance, do not actually fix this problem. Her E is probably the biggest culprit here. On paper, she gets to have a massive damage execute after AAing someone for a while, that resets on kill. In practice, she has to pop it early for the slow so she doesn't die to the enemy garen diving her, which disables the stacking passive. By the time it comes back up again (8 seconds later), she's spent a ton of time AAing things, but nothing has any stacks, so the "execute" does nothing. Twitch's E is also on an 8s cooldown, but his AAs always stack poison (which does deceptively large amounts of true damage in the mean time, might I add). When it comes back up, he'll probably have a max stack target to pop E on again for some extra burst.

Rend not continuing to stack after using it doesn't just gut her damage. It also feels terrible. You will almost never be able to use the move in an optimal way, and it feels like you're always handicapping yourself no matter how you choose to use it. If you save it purely as an execute, you lose out on an important slow in her kit that could mean the difference between surviving a teamfight and getting dived on. If you use it to prevent someone from diving you, your damage is gutted for the rest of the teamfight, because unless you somehow have time to be AAing a conveniently placed nearby enemy minion while getting dived, you won't get the reset. If a vayne has to use her E to keep an enemy at bay, all she loses out on is the possibility to stun them by hitting them into a wall. It still feels good to use, and it doesn't compromise her kit. In kalista's case, if she has to use her E to slow an enemy, she loses (27 + 27% AD) damage on every AA thereafter, which is a huge amount of damage.

If letting her continue to stack rend after using it is somehow still not enough, another possibility to look into is giving her a damage oriented passive on her ult. For example, something simple like 10/20/30 bonus AD, so leveling up her ult does something besides just reduce its CD (90/75/60). Perhaps triggering her W passive could let her oathsworn gain the same bonus or increase the bonus for herself for a period of time.

tl;dr: Let kalista continue to accrue rend stacks even when rend is on cooldown, and see if she feels better to use after that.

edit: A clear visual indicator on when her E will kill a target would be nice, too, so you know if you'll get the reset.