Wolves don't follow the spawning rules of passive mobs, because they aren't passive mobs. They're classified as neutral mobs: mobs that are aggressive, but only after being triggered by the Player. If Wolves were made persistent, angry aggressive wolves would also stay in the world.
I don't see why that's an issue. Can't Mojang simply change their spawning mechanics to where they only despawn while angered? I mean it doesn't sound very hard to do. Just add an exception/check to see if the wolf is passive or not