> Without external restraints, any sufficiently large company will focus on eliminating competition either through pricing them out of the market or just outright buying them.
The counterargument is that this only happens in markets with high barriers to entry, which in turn are mostly caused by regulatory barriers.
For example, the government makes permanent spectrum auctions, so if all the holders of spectrum merge together, no new competitors can enter. Suppose instead they held a new auction every year or every month for use of the spectrum in a given region until the next auction. Then mergers wouldn't prevent competitors from entering; a new competitor goes to the next auction, places a bid and can get spectrum to launch a competing provider.
It's clear that this theory is sometimes true -- that regulatory barriers can do this. Whether it's always the cause is hard to say and depends mostly on how you define what "regulatory barriers" means. But consider the consequences either way.
If there are regulatory barriers causing monopolies, you have to break them up. If there are some other things causing monopolies, you have to break them up. Monopolies are bad. And breaking them up, if they are caused by regulatory barriers, can hamper the entities lobbying to keep the regulatory barriers.
The question is what comes after. You don't want them to just reform again, right? Antitrust is a necessary evil, but still an evil -- still means you had a monopoly for long enough to have to bust it up. So if it's mostly or entirely regulatory capture which is causing them, we still need to address that. And then we don't need antitrust enforcement as often.
But we still need it whenever a monopoly/oligopoly forms. That's just not a thing that can remain.