EasyJet was once all 737s and a large and important 737NG customer with dozens of 737-700s online before most legacy carriers had some, and they eventually went all Airbus. Allegiant has been all Airbus for about a decade and is taking the risk of a 737MAX future (which paired with the massive expansion of Sun Country and their transition to a ULCC makes me suspicious of things behind the scenes). Even on a scale of hundreds of frames, it is entirely possible that delays and issues could get a long-time Boeing customer to go Airbus (especially in the case of UA where what is adding an extra 100-200 A321NEOs and canceling MAX orders when you already have 100+ A321s on order?).
That said, I don't see Southwest ditching Boeing. What I could see, since the WN of the past is already a memory anyway, is something like the A220 or E195-E2 or something becoming a 737-700 replacement and operating alongside the 737s in large numbers. Southwest may say, for now, that they'll just "wait" and make due with those 737-700s that are going to hitting their max cycles in the not-so-distant future, but I don't buy it. If it turns into a 3-5 year wait, planes need to go to the desert, and Embraer and Airbus offer the right deal, anything could happen. All things considered, it isn't THAT crazy to think about given recent events at Boeing and Southwest moving into markets like SBA, FAT, EUG, BLI ect which it historically wouldn't have considered wroth the trouble.
Not to mention carriers going bankrupt or canceling orders. Braniff became the 1st A320 operator in the US because Pan Am had to cancel the order right as the planes were coming off the line so suddenly those first planes were up for grabs. You never know.
