You are right. The FCC can't grant powers to itself. If it does overreach, it gets spanked in the courts. So it is unclear what you are frightened about... the checks and balances are in place, and by your own example, quite effective.
The current rule making, as all successful rule making, is based on EXISTING AUTHORITY ALREADY GRANTED BY CONGRESS. The FCC tried to use its existing authority to create net neutrality rules. The court didn't like how they justified their authority so they sent the FCC back to the drawing board with reasoning that forms a roadmap for the current iteration of the rules. That was version 1.0. This is version 2.0.
Consider the pace at which Congress is able to pass laws. It is glacially slow. In our system of government, enforcement and interpretation are left to the executive branch and the courts. It is the only thing that makes the whole system work... that other branches closer to the practical problems of implementing the law are left to "fill in the blanks." Congress INTENTIONALLY leaves those blanks because it cannot predict every eventuality.