Earlier quoted context omitted.
I'm not an expert by any means, but this is almost the opposite of what I've heard, and the fact that most airlines are either Boeing or Airbus exclusively supports that idea. In particular, I've heard that maintenance and pilot training are big issues. If you've got half your pilots on the 737 and half on the A320, you're much less resilient to scheduling issues than having all of them on one or the other. Similarly…
The only airlines that are Boeing or Airbus exclusive are low costs (like Ryanair or Spirit Airlines) and tiny airlines operating a handful of planes. Otherwise it's all mixed fleet (Delta, Air France, JAL, ANA, Lufthansa, UA, etc.). If you separate wide-body and narrow-body then your statement is more correct: as going narrow-body Airbus and wide-body Boeing (or the other way around) is not rare. The threshold I've…
This isn’t true for the reasons you are implying. Most of these smaller carriers fly to regional airports with smaller runways and smaller demand which the 737 specifically is ideal for (since that’s what it was designed). It’s also one of the most common planes out there, so when smaller carriers are looking to buy, that’s what’s available. It’s not that they are looking to be a single vendor airline. It’s just that the one vendor made the plane they need.