I'm being vague, but I think my concern is that the focus on "monopolistic behavior" is archaic. Harm to a healthily competitive environment can result from a few big players existing rather than a single dominant one. It may not even require certain "behavior" as much as a large entity with lots of capital simply existing. There are also big barriers to entry in any business that is built on network effects - even there, we need a mechanism to enable competition.
>I'm being vague, but I think my concern is that the focus on "monopolistic behavior" is archaic.
I take your point. However, at least in the United States[0], the law does not require an entity to be a monopoly to engage in behavior that is anticompetitive and/or violates anti-trust laws.
In fact, the topic at hand (Visa's proposed acquisition of Plaid) doesn't have a "monopoly" component at all.
Rather, IIUC the DOJ is arguing that it runs afoul of section 7 of the Clayton Antitrust Act[1] which addresses mergers and acquisitions that limit competition.
I don't disagree with your assessment of the situation, but I do think that existing law (and jurisprudence expanding on that law) address the vast majority of the issues impact competition today.
