Earlier quoted context omitted.
That doesn't sound likely. They can wait for existing planes to age out and then buy a competitor's planes. For many reasons including fuel efficiency, when an airline retires a plane they are not going to replace it with the exact same plane. So some levels of pilot retraining is expected.
Type certification is a thing that exists, and the whole reason the 737 max is designed in this particular wonky way is so that it can grandfather into the type certification from the existing 737 family. https://en.m.wikipedia.org/wiki/Type_certificate Having to mix and match pilots across the fleet is less efficient, what if you have a 737 pilot but the only aircraft available is an airbus? So, many airlines (eg so…
And yet many airlines fly a very high amount of planes of many types, of multiple brands. Southwest is a consumer darling and punches above its weight but it is an outlier.