Do Riot actually think that Kalista's kit is healthy enough to be properly balanced by numerical changes alone?
Her passive makes her the definition of a binary, hard-counter champion. Her enemies need to have one (or preferably more) of the following to be able to do anything against her:
- Targeted or instant-AoE CC (root or stronger)
- Heavy burst damage to delete her before she can react
- Multiple gap-closers (or a single gap-closer combined with burst and/or cc)
- Ranged non-skillshot damage at a longer range than her autoattack range (marksman autoattacks count here)
Any champion who doesn't have one of those has extremely few options to deal with her. They will lose any fight unless an ally with the above can either lock down Kalista for long enough for them to interact with her, or burst her down quickly enough to take her out of the fight.
This means that she tends to win or lose games based entirely on which champions the enemy team have picked.
While that tends to make her roughly "balanced" in terms of win rate, it directly contradicts Riot's own stated Design Values:
League isn’t won at champion select and no phase of the game should be decided before a team steps up to fight.
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A fight loses its excitement if the first punch wins – it's what you can do after that adds depth and complexity. We try to prevent hard counters and abilities with no room for reaction, as no game should be over without a real show of skill.
Judging by the many threads that have been made about her here on the boards, most people seem to agree that her passive hopping mechanic is simply not fun to play against if you happen to be one of the (large) subset of champions that she excels against.
Outside of the gameplay and balance issues, her theme is a vengeance spirit that works together with her soul-bonded invoker to hunt down and punish her target. All of her other abilities fit towards that purpose, but her passive doesn't.
A vengeance demon that hops around like a rabbit, and mostly uses those hops to rain down death while running away (and being uncatchable) doesn't fit that theme at all.
Her kit has enough unique abilities in other places that she would still be an interesting and usable champion even without her current passive.