I never thought the branches are co-equal. Congress controls the POTUS, if they want to. We just saw four years of Mitch controlling who Trump chose for his Admin. Congress can impeach Justices, can change the number of Justices, or their jurisdiction. Legislative is the lead branch, then Executive, then Judiciary. Trump just found out the POTUS is not the most powerful man in the World.
Agreed, co-equal in theory only. With the power of the purse, the authority to impeach and remove presidents and cabinet officers, etc, it’s small wonder the legislative branch is Article 1 in the Constitution.
To your point:
One could argue that Congress has slowly encroached on the Executive by excessive oversight. There is no specific oversight power of Congress, but it is implied by the "necessary and proper" clause of Article I Section 8:
To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.
Congress interprets this as needing to oversee the proper execution of the laws that are passed, but then they stray beyond "necessary" execution into other areas.
Oversight is also implied by the impeachment clause, suggesting that Congress is always on the lookout for impeachable behaviors (treason, bribery, or other high crimes and misdemeanors).
Oversight on Congress is implied by the Article I Section 6 privileges clause:
They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.
The Executive can bring charges against a Congressman for felonious behavior or "breach of the peace."
Interestingly, "breach of the peace" seems to be just as vague a term as "high crimes and misdemeanors" when it comes to judging actionable behavior, but I don't see anyone claiming breach of the peace when Maxine Waters calls for harassing people on the streets.
It seems that these days, Congress sees anything a Republican President does as impeachable, and nothing that it does as a breach of the peace, including supporting and bailing out rioters and encouraging the intimidation of people and driving them out of restaurants.
-PJ