Boeing are going to rename the MAX so a lot of consumers won't even know they are flying a MAX. From memory Ryanair have already named their model the 737-8200 (which is a semi custom version of the MAX-8 just for Ryanair where the toilets are just oversized shoeboxes to fix an extra row if memory serves).

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