These companies aren't quite monopolies, but the fact that they are "too big to fail" seems to imply that they are involved in a form of market manipulation, whether intentional or not.
We seem to have forgotten that the original purpose of antitrust law was also to prevent companies from becoming too powerful. Too powerful in that so many other companies depended on them, so many jobs turned on them and so many consumers or investors or depositors needed them, that the economy as a whole would be endangered if they failed. Too powerful in that they could wield inordinate political influence of a sort that might gain them extra favors from Washington.Maybe the biggest irony today is that Washington policymakers who are funneling taxpayer dollars to these too-big-to-fail companies are simultaneously pushing them to consolidate into even bigger companies.
The prospect of corporate mergers leads me to ask "what is a corporation": it is a legally binding agreement among producers to regulate the production, pricing, and marketing of a good. This is the definition of a cartel, except that a corporation is treated as a single legal entity rather than a collection of entities. The fact that the maintenance of the corporation is legally enforced (shareholders cannot withdraw their capital) is important, because cartels are unstable in the absence of legal enforcement.
The very existence of a corporation (a creation of the state) implies the need for rules regulating how large a corporation can be. We've become very lax in this regard, and I think we're getting screwed for it.