This is such a pervasive argument also used to justify a lot of bad stuff in tech ("if we don't do it, our competitors will" / "if you don't like it, you should have asked the regulators to stop me").
This is a small world. The US has something like ten major airlines. Eight if you don't count subsidiaries. Someone at the first one made a decision to embrace an exploitative pricing model. And then, someone at the second airline looked at this and thought to themselves, "you know, we're not gonna organize a PR campaign to stigmatize it, we're gonna try the same".
I'd argue there's only three "major" carriers: American, United, and Delta.
The rest, such as Frontier, JetBlue, Southwest, Allegiant - are all discount and/or regional airlines.
For example, allegiant flies to most cities secondary airports that are 1-2 hours drive from the city.
EDIT: It's really 4 equally large ones by passenger miles:
* 1 Delta Air Lines 17.8%
* 2 American Airlines 17.5%
* 3 Southwest Airlines 17.3%
* 4 United Airlines 16.0%
The rest are sub 10%
Free markets compete, it's what they do.
It's like getting mad at your cat for killing a bird - it's just what cats do.
The answer to this behavior isn't being mad at or boycotting particular airlines, it is regulation.
My point is that it's totally unclear to me that that's how this happened. There were at least a few airlines that did not say "we're gonna try the same", such as People Express Airlines and ValuJet, which explicitly tried to market themselves as fair, cheap, and transparently priced. Those airlines don't exist anymore. They went out of business, mostly because the other airlines used sophisticated pricing schemes to sabotage them. Maybe this was a failure of antitrust. It's certainly much more complicated than "all the airlines colluded together". Given the fact that the surviving airlines were never able to reach a particularly profitable "recoupment phase" of the predatory pricing process, it seems that this did not actually result in unfair prices on average (although business customers obviously get shafted every time).
In other words, modern airlines gouge certain customers, but effectively subsidize others. I would be unambiguously outraged only if they were gouging everyone and making fat stacks.