Delta's answer to the ultra-low-cost-carriers, 'Basic Economy' fares is working so well for the airline its planning to extend the option to more domestic markets. The other legacy carriers are following a similar path, but Delta has been the leader, and first to report.
The fares are lower than the regular economy, or coach, fare passengers are used to, but there's a price. No seat assignment, no changes, no upgrades even for a fee, and fees for every other extra. The fares are often 20-25% cheaper, and are aimed at competitng with carriers such as Spirit, Allegiant and Frontier.
Delta attributes $20 million in extra revenue in the first quarter of 2016 from filling those seats with passengers who might have booked otherwise on a true ULCC. In effect, you now have three classes of economy on Delta, the basic, the regular, and a premium economy that comes with a bit more room and amenities. Delta believes its service reputation gives it a boost over the ULCCs.
The extension to more routes will come mainly where ULCCs are strong competitors, and not on routes where they are not.