This always seemed like a cop out to me. There is no way that the Founding Fathers needed, nor intended to anticipate changes in the future. They wrote the Constitution in such a manner that it could be used to justly rule a country regardless of the existence of Youtube, Ponzi schemes, an illegal drug trade, etc.
I think you are absolutely correct that the way an agency is run depends on who is in charge, but because people aren't being thrown into prison for the way that they run their agencies, things aren't going to change. I'm trying to find a story I heard on NPR the other night to post here, as it exactly illustrates both your, and my point, but I can't seem to find it.