Subordinates always have a certain measure of power over their superiors. Sometimes idiots come back to the office with a gun because they're mad they got fired. However, when the US Constitutional system is working as it was intended to by its framers, Congress is the preeminent branch, precisely because it requires a great degree of consensus to exercise that power, unlike the other two branches. The bicameral legislature really is a great design.

Also it would be pretty amusing for the Supreme Court to attempt to rule that impeaching a Justice is unlawful. I'd love to see the creative "interpretation" in that ruling.