SABRE is ancient technology, but very reliable and at the same time extremely inflexible. Last time I saw it upclose in the early 2000's much of the core was still coded in IBM assembler, although over the decades more pieces were slowly being modernized so I have no idea where it is now. Sabre is a horrifically un- imaginative company where projects are measured in years and not much every changes.

I think though Southwest's issues are more on their side.

Yeah building a new GDS today is an exercise in insanity, it's a huge complexity nightmare and switching probably impossible. I always wondered if AI could eventually improve things, but the existing GDSs are unlikely to care much to try. It's basically a (tri)monopoly you can never break.