Looks like the set bonus is here to stay. As someone who already has the entire set including a shadow with +10 discipline, you can say I'm somewhat pleased. I foresee an incoming massive spike on the prices of the pieces.
We're not going to change/correct the set bonus of Natalya's. It will remain as-is.
It's one of a few cases recently that made us really sit and look at how we were approaching a game system, have some serious discussions about what they mean for the game, and where we want it to go. Sure it's one set and set bonus, but allowing it to continue existing, and even now beginning some work to embrace the concept, probably wouldn't have happened if the unintended change to Natalya's hadn't occurred, and maybe more importantly if you didn't offer your feedback (constructively and in calm and reasonable tones, of course).
We are working on Legendary item changes for a future patch, as I'm sure you know, which could change the set in any variety of ways. I don't know what those changes could be yet, but I do know there's no desire to "fix" the set, even in the updated version. But regardless those changes will only affect items that drop after that patch. They aren't retroactive. Any Natalya's items that drop before that patch (ie the ones dropping now), and the set bonus, will remain unchanged regardless of future Legendary item updates.