Boeing is at the end stage of the feedback loop where regulatory failures lead to monopolies and vice versa.
The regulatory failures usually come first, either a failure of antitrust (they're allowed to buy up the competition) or a regulatory environment that itself puts the competition out of business. Then you have a consolidated market.
Monopolies are like cancer. They metastasize. If you didn't have regulations that keep upstart competitors out of the market before, the incumbents will welcome them if not actively lobby in favor of them. And then you're stuck with them. They're too big to fail. If you had 10 other domestic aircraft companies and one of them was screwing up, they'd not only have to contend with customers going somewhere else, the government could actually punish them. Whereas if there's only one, what are you going to do? Bankrupt the only domestic supplier? And they know it.
The only way out of it is to restore competition. But the longer you wait, the harder it gets to fix it.