Anti-monopoly failing is the effect, not the cause - how is there a monopoly to start with?

You may be poorly educated as to how antitrust is supposed to work.

Breaking up monopolies that already exist is only one of its purposes and tools. It is also there to prevent mergers that will be detrimental to the public.

Past a certain (very low) scale threshold, this should be assumed to be "almost all of them". Rather than the government having to prove that a merger would be detrimental, the companies wanting to merge should be required to prove that the merger would be beneficial to the public in long-term ways that do not require specific (and easily breakable) "commitments" on their part to do things like keep prices low.