So what your saying is that the salary cost of 1 FA outweighs 12-21 more seats? Rather have high frequency flights than cut back on flights and save fuel. They operate the 500 and the 300 so why not add seats. Plus the fuel burn for an 800 is better than a 500. There are routes where WN can fill up a 800 no problem on a regular basis. The only additional cost would be that 1 FA. Training and ops cost should be the same based on the passenger load.
Looks like I'm late to the game, but here's what I typed before I saw some of the same responses.
Southwest has an entirely 137 pax fleet (500+ a/c), with the exception of the 25 -500's on property. One of the problems this helps us avoid is an equipment downgrade, which adds pax re-accommodation costs, should an a/c experience a mx event.
Also, we're enjoying that nice fuel burn you mention with the -700's, which have a greater flight range than the -800.
AND, it adds a layer of complexity to schedule a line of flying for a separate a/c type, plus crewing, plus possible contract language, plus.....
All this together is why the -800, so far, isn't in the plans. I reserve the right to be corrected by Management decision, in the future.