It stopped forcing airlines to provide service to regional airports, and paved the way for the consolidation of the industry into the monopolists as Eastern, Midway, Braniff, Pan Am, Continental, Northwest Airlines, and TWA were liquidated.
4 "major" carriers - American, Delta, United, Southwest
1 "mid major" / borderline major in Alaska Airlines
1 "not a major, but not exactly a regional carrier" in Jet Blue
and a bunch of regional outfits like Frontier, Porter, Spirit, etc.
That's not to say that more competition wouldn't be nice, but it's hardly the case that there is no competition. Heck, as far as that goes, Peter Thiel uses airlines - and their razor thin profit margins - as the prototypical example of why "competition is for losers"[1] in his talks.