They had Boeing design a bespoke 737 variant to simplify operations and maximise throughput.

Neither does Ryanair.

My friend works in the quality control on the production line. They build the same thing for multiple clients. And they have different quality standards for each of them. Some parts would be considered faulty by one of them but good by another, so my friend needs to move faulty elements to the batch for the client that will accept it.

Of course the client with the highest quality standard is charged a lot more than the client with the lowest standard. Maybe Boeing didn't make the engine, but they definitely negotiated the price of it.

Of course I don't know if something similar happens in the aviation.

I flown Ryanair multiple times, and I had to put my legs in the language compartment, because human physiology seems too expensive for Ryanair, but I'm annoyed seeing how quickly the entire internet forgot the Boeing's quality issues, so I'm here to spread doubts.