In short if you want the ~15% improvement in fuel efficiency of modern engines with the 737 airframe you need a MAX. It is bordering on impossible for Boeing to take the 737 airframe any further after the MAX.
Also, it's not just the 15% decrease on fuel burn (which obviously doesn't equate to a 15% cheaper flight) but Southwest was the launch customer for the 737 NG series which means their initial 737-700 planes are now 24 years old, which means increased maintenance costs; a 24 year old plane is probably coming up on its second (or third for planes that have a higher duty cycle) D check which would be approx USD1.1~1.8M depending on the amount of remedial work / engine replacement etc. So avoiding a D check can save a lot of money. Multiply that by the 495 737-NGs that Southwest has and it is a very big number.
Here's a link[0] that gives an idea of how modern commercial aircraft checks work.
[0] www.aircraftmonitor.com/uploads/1/5/9/9/15993320/basics_of_aircraft_maintenance_programs_for_financiers___v1.pdf