They say that that Southwest has run out of low-hanging fruit, out of secondary airports like Oakland, Cal., or Long Island, NY's MacArthur, and that's why it's going into real big airports where they have lots of old-fashioned airlines, airports like Minneapolis/St. Paul, where it starts flights in March.
True, but only up to a point. Consider the LUVline's plan to enter Boston's Logan later this year. It hasn't chosen an exact date and it hasn't named routes yet, but Southwest is entering a very different Logan than the one it shunned back in the 1990s when it began flights to Providence, RI, (1996) and Manchester, NH, (1998), straddling Boston with two secondary airports offering easier ground access. But since then, Boston is has changed. Not only is the airport easier (or least less difficult) to get to with the finally final completion of the city's 'Big Dig' road project.