Kalista's win rate is 47% and she's getting nerfed...
...Meanwhile other ADCs like Kog, Twitch, and Tristana have had high win rates for a while now and aren't touched?
There is no reason why Kalista needs to be nerfed, except MAYBE for pro play. Even there I don't think she is that problematic, although admittedly I haven't been following LCS closely this season thus far.
If that is Riot's reason, then that would just reinforce the belief that Riot heavily balances the game around pro play, often at the expense of the general playerbase.