What everyone has said is accurate!
Flying into a JFK or LGA ect, would have an adverse effect on WN's bottom line. And for an airline that prides itself on the famous '10 minute turn'(20 minutes @ LAX) this would not be possible on a regular basis at any of the airports that you mentioned. I just mentioned LAX, now that Steve has mentioned these big airports, I have always wondered why WN flies into LAX, when it seems to go out of its way to avoid the majors, perhaps someone can answer this. The only other major that I know that WN serves where there is a competing airport that WN flies into is IAH. Of course WN has a huge operation at HOU some 30 miles to the south of IAH.
WN use to fly to DEN up until the mid 80s, one of the reasons I was told (among others) that they pulled out was due to delays, in particular weather delays.
In any case Southwest's formula works very well and hopefully no one will tamper with this 31 year success story......"If it ain't broke, don't fix it!"
"Show me the Braniffs"