(Really? LAX is not at capacity most of the working day? Or do you mean they ration slots via some other method?)
It’s a free market with many caveats;
first, the barriers to entry are quite high as montioned above - so competition is limited there.
second, the difficulty and cost to add a route or increase capacity are high, even once the carrier is established.
third, the market is extremely volatile, so opportuities for real profits are risky.
Most airline markets are oligolpoly, or usually “follow the leader”. Cutting prices is a zero-sum game; if you undercut your competitor, they match, you both lose; it’s easier to match the leader or undercut just a little; or try to compete on lower-cost amenities. Occasionally an upstart will try to compete, forcing the established companies to match ofr a while creating a temporary bargain for consumers.
Spirit is playing a shell game. Many consumers now shop online and are very price-sensitive. They expect to attract consumers with a lower price, then hit them with an extra fee, too late, when they board. That will work for a few months until they catch on, then the new campaign will be “plus one free carry-on!” and it will look like a bargain.
They are also playing fast and loose with the rules. The rules say you must advertise the actual price, so they can’t hide things like “fuel surcharge” in extra fees. Since you don’t *have *to have a carry-on (or luggage), technically this is an optional charge, and doesn’t appear in that all-so-important expedia/travelocity price number.
…Which may eventually cause the government to make the rule “the price an average consumer would pay”, i.e. someone with one carry-on and one checked luggage. Until then, they’ve got a false price advantage with online shoppers.
Does it really cost the airlines anytihng for carry-ons? No, if they enforce the size and weight rules so there’s enough space. Charge for checked luggage, and people push the limits of carry-on to save money. Charge for carry-ons, and people will find coats with super-big pockets. Charge for clothed passengers, and nudist resorts will do a booming business.