The problem lies in the very real fact that once these apparatus' gain their charter and/or their freedom of action, they will not voluntarily revoke it. Power gained must be maintained, even if it involves conjuring threats to justify their existence.
Small fish get eaten by big fish, and thats what will happen with time. Just as the OSS became (or was replaced, depending on how you look at it) with the CIA, so will many of these current agencies eventually be absorbed by the central power from which they derive their authority, namely the state, the biggest fish of all. They won't have any say in the matter, they will go voluntarily or face the consequences.
Our nation is not at risk of falling asunder to terrorists attacks. Our institutional reaction has proven to help them succeed beyond their wildest expectations.
For the state, terrorism is merely a means to an end. The real threat you face, and probably why these many agencies have suddenly mushroomed is because 21st century realpolitik
requires the USA to be all powerful in any and every sphere of conflict. Since the state requires the ability to kill its enemies, whilst maintining a politically acceptable veneer of civilisation, then terrorism is a convenient excuse to build up a 'new model army'.