My understanding was that they’re trying to use software to make their new planes fly like their old planes (at least that was the reasoning for MCAS in the 737 MAX). In super basic terms, if the new plane doesn’t fly like the old plane, all the pilots need to be retrained. This is a significant expense that could make a serious dent in sales.

I don’t think it’s anything to do with size, there are larger planes than the 777X that are perfectly safe (and of course, the 737 is pretty small as far as airliners go).

Isn't there also some weird requirement that pilots only fly one/two airframes at a time? So training for a 777 means your can't fly 747 or something else anymore?

I don't think so. But it was easier to sell to airlines if it didn't require retraining.

It seems like there's a very strong incentive to avoid retraining coming from somewhere, at least. Surely that's not just airlines being stubborn?

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.

The Antonov An-225 Mriya was designed in 1985. I'm not an expert, but I doubt it has any significant computing power on board.

https://en.wikipedia.org/wiki/Antonov_An-225_Mriya

The 777 is fly by wire so it wouldn't be a good idea.