With the Mage and Paladin nerfs there was a possibility that Control Priest could become an oppressive meta-breaker (...)
Priest, with some effort, could already beat Paladin pretty consistently and in NM Mage matchup +1 mana on RSW will change absolutely nothing.
The Mage-Paladin meta was annoying, but a potential Priest-Warlock meta would be disgusting. This is a proactive nerf.
Priest-Warlock meta? Priest still has some terrible matchups and even 0 mana Hysteria wouldn't fix them. Warlock is atrocious in general, so no, that would never happen.
Priest's only really weak matchup is Warlock. The reason the class was dropping winrate against other classes was the attempt to tech against control, thus losing the edge it had against aggro and face decks.
If Priest is allowed to become strong across the board, then we might get a meta where players are either playing Priest to beat everything except Warlock, or Warlock to beat Priests.