It was hilarious.
You see, Shadow Vulnerability used to be a debuff on the enemy. And it affected other players' damage as well. So the effectiveness was cut in half, so the Warlock damage was nerfed. By nerfing a Shadow Priest mechanic.
Never, ever assume Blizzard to be logical or fair. Blizzard wouldn't hesitate to nerf Shadow Priests if Dark Intent was the sole cause of the imbalance.
Generic on-topic: Statistically, someone has to be #1 on DPS. It's basic logic. The notion that #1 does too much damage, and should be nerfed, just means next patch that someone else becomes #1. Does that person then need to be nerfed?
When there's a margin of about +/-10% DPS separating you and the next highest DPS, that's fine. When you are consistently 20% above everyone else, that's grounds for a balance adjustment.