Well the big benefit of the MAX is the engines, and the new engines have a much bigger fan, which means a complete rearrangement of how the engines are mounted on the wing, and how that affects the wingbox, which means in effect yes, they need the MAX. Boeing before the MAX faced the dilemma of the successful 737 form factor, but the requirement for larger fans for more fuel efficiency. They had to toss up between a newer replacement model (complete with more ground clearance, new design which is about a decade long timeframe, with the osbourne effects etc) or shoehorn bigger fan engines on the 737, which is what the MAX is.
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.
They do need it, they are a low cost airline that gets its main cost benefit by having only one supplier and one airframe. I guess you could say they don't need to be a low cost airline as well, but that statement would hold the same amount of weight as saying they don't need the MAX.
i'm sure if Southwest offers 15% discount to fly on MAX, vs regularly priced tickets on regular plane - nobody is going to choose MAX.
Consumers, in aggregate, are way smarter, than the corporate folks
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.
