Just as well! Oligopolies are almost as bad as monopolies, for rent seeking and stifling competition.
I don't know about US law, but EU competition law is concerned about “significant market power” and not (just) monopolies.
When talking about modeling oligopolies and monopolies, often oligopolies have worse results.
We need more antitrust on everything. Things are not working if consumers have no choices in practice. "Support Nestle or just never purchase any snack or drink," isn't a real choice.
It also isn't a reality, and if it was, it would be a monopoly, and it would be dismantled.