Pick a random origin or destination airport: which domestic carriers are you most likely to find at the selected origin or destination airports?
I mean, Mesa is a domestic carrier, but you're not going to find them at a significant percentage of airports. Similar for Jet Blue and some others you listed.
The small airlines thing used to trip me out when I was flying nationally on a regular basis. Was often a case of "yeah, it was ticketed through <AIRLINE> and the plane bore their livery, but the flight was actually operated by <REGIONAL_AIRLINE>"
And, while I loved JetBlue, they rarely went to the airports I wanted to go to. Worse, they never left from my closest airport (<10mi from my house), meaning I'd have to catch a $75 cab to get to the airport they DID operate out of.
Being someone that flew for work in the 90s – when we still had a bunch of carrier-options – prices were much higher in absolute (rather than inflation-adjusted) dollars than now. Planes and airports were both far less packed and major weather-events didn't create the kind of chaos that this year's did. Maybe this is one of those places where we really don't want to exacerbate things by a race-to-the-bottom on prices (services, etc.).
@rbreich
There is a clear lesson in the Southwest debacle about concentration.
Southwest is a niche carrier, and look at the turmoil they created with a system failure.
Now, imagine that was Delta.