Retraining is costly not only in pure financial terms, time is also a major issue as at any time in the world there are only X available simulators to be used for retraining. If you have hundreds of pilots in need of retraining, using a sim for 20-40 hours while you only have 50 spots available it will take awhile before you can start flying your new fleet, downtime for these pilots in training is another major cost.

There is no level playing field here, if pilots were required to be retrained for new planes even if they are the same class it'd increase costs in general but it would, at least, level the field to avoid these shortcuts for certification.

Being hyper efficient has its costs, this is one of them, an airline can't stop flying some of their pilots because they are on the razor edge to keep their margins. It is in the design of a competitive market, we might have to take a second look on what's the net-positive of having that much "efficiency" forced by the lack of incentives in other areas such as safety.

It's not just stubborn, it's really expensive.