Earlier quoted context omitted.
well, as an example, when the 737MAX was grounded, many airlines had already started taking delivery of them and were operating them. Southwest was the largest recipient at that time. Southwest had plenty of older 737s as well, but the new MAX planes both fly farther and have more seats, so while the grounding was in effect Southwest was essentially paying to keep the things on the ground while scrambling to reconfig…
Single AC fleets mean an airline has somewhat of a single point of failure. If the issue at hand is caused by that point of failure, sure recovery will be harder. In most other cases it makes it actually easier, at least on paper. No need to get crews to certain planes, just a crew to a plane, as every crew is certified on any plane.
If plane going from A -> B is late it cannot become flight B -> C. If crew for this plane are stuck in the ground long enough, they have to wait til the next day to fly as they‘re only allowed to be awake and work for so long.