I suspect that a lot depends upon the deals that airlines get. I'd give very good odds that IAE got a very competitive rate with their recent Max order (or letter of intent). The loco model, certainly as applied by Southwest & Ryanair works best with a single type - variants of the type are ok as long as you have a single type rating. Once an airline has committed to a type it makes sense to stick to it unless a manufacturer makes a killer offer - which I suspect Airbus did with easyJet.