How is it possible that Joh Fredersen didn't know that destroying the heart machine would cut power to the entire city? Isn't he supposed to be the visionary mind that brought the city to life. It makes sense that he wouldn't know the minute details of every machine (that being more Rotwang's department) but not knowing that the heart machine is the cities power plant (or at least that it is closely tied to the power plant) is a little hard to believe, especially when the moral of the story suggests that Fredersen would know how the machines work and the workers would be ignorant of this.
The screen Fredersen uses to contact Grot is labelled "HM 2", which suggests that the Heart Machine destroyed by the workers isn't the only one. He may have intended to break the workers' spirits by allowing them to destroy their own city, followed by a crackdown and work moved to another Heart. Unfortunately, the apparent deaths of the mob's children drove them into a rage instead.