It isn't so much that it breaks the game, but it creates a lot of bad play experiences. You're right, you could just do Fold Reality on other units...but before this book, where was the value in doing so? All those units you listed were never considered for competitive play in the previous battletome, and the Daemon keyword carries a lot of weight nowadays. Skyfires and then Enlightened were just amazingly efficient units that with that Keyword, could just mitigate a lot of their flaws. Fold Reality regenerates the unit. Locus makes them harder to kill in combat. There's a multitude of ways of getting -1 to hit in shooting between some spells and Eternal Conflagration. Taking it away let's them keep certain avenues of mechanics open for the army overall without it getting out of hand and creating a one dimensional archetype.