I think Southwest is very opportunistic. Assuming aircraft and crew availability, rather than merge I suspect WN is more prepared to pounce as opportunities arise. They did that a decade ago when USAir essentially abandoned the north/south intra-California routes (plus LAS and RNO). Haven't there been reports about WN expressing interest in routes (the East Coast Shuttle) that DL might have to divest if the US/DL merger comes to fruition.

As I speculated in another thread, if merger mania comes to the airline industry, it will impact WN -- in the form of new opportunities to continue what they've been doing very successfully for a long long time.