You're right that switching airplane models certainly seems possible, even for airlines that operate one model exclusively. I wonder if Southwest being such a huge airline by fleet size (much larger than EasyJet for example) would make that harder or easier.
Either way I was thinking less about Southwest jumping ship to Airbus and more switching to an entirely new model from either manufacturer. I wouldn't see Southwest going through the trouble of switching to the A320 for example, but maybe to an A320 or 737 replacement. The A320 may be a newer platform than the 737, but it still dates to the 80s (admittedly better than dating to the 60s). I doubt Southwest would want to go through the hassle of changing models just to end up on a 40 year old airframe that they might have to transition away from again in the not-too-distant future.
Either way I was thinking less about Southwest jumping ship to Airbus and more switching to an entirely new model from either manufacturer. I wouldn't see Southwest going through the trouble of switching to the A320 for example, but maybe to an A320 or 737 replacement. The A320 may be a newer platform than the 737, but it still dates to the 80s (admittedly better than dating to the 60s). I doubt Southwest would want to go through the hassle of changing models just to end up on a 40 year old airframe that they might have to transition away from again in the not-too-distant future.