Granted, the rollout across airlines is probably glacial
Source: I used to work there
Complexity — Fundamentally you’re looking at a logistics software, except unlike packages you’re dealing with people who aside from expected destinations have travel lengths and time-in-air calculated down to the minute. Also unlike a package, a surprise multi-day trip, unexpected multi-leg journey, one day delay is not something passengers (and crew members) will accept or be at all ok with. And if any one thing goes wrong there’s going to be cascading failures down the line— so much that it may break your company’s entire operating workflow (e.g. Southwest) entirely, and no software can overcome that kind of organizational gap.
Airlines - There’s not many commercial passenger airlines left in the US, especially that fly nationwide. Good luck trying to convince one of these giant behemoths to move to a non-battle-tested system for core operations, especially when decades-old industry software and practices around that software exist.
Also— Sabre’s software itself is actually reliable! As a corporation it is slow clunky and bureaucratic, but the actual functionality it provides is stable, battle-tested, can handle any travel edge case you can think of, and fast and efficient for those who know how to use it, while also good enough at day to day operations that it doesn’t take too much time to train new agents on how to use it for routine tasks.
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.
1) Replacing any software for an airline carries huge risk. They are barely operating ok with the software they have and holding it together with duct tape. Even something you might regard as ancillary, like a baggage handling software system, or flight catering software system, if it goes down, has the potential to disrupt thousands of planes and hundreds of thousands of passengers for days.
It is so significant an issue (to try to change some software, and just one out of many systems that have to talk to each other) that if an airline ever considers doing this, they may actually stop operations for some number of days while they do it rather than risk having operations go wrong. There are some rare examples of airlines doing this to try to change their systems.
2) Related to the above, airline management hates to be embarrassed by something that might work but has the potential to go badly wrong. So they are very conservative when it comes to replacing systems that are working, even if it's painful / much less functional than what they might achieve by a change.
Combine these factors (and many others) and it means that sometimes starting a new airline is simpler than trying to fix an old one...
Now there's a truly total rewrite...
Let's ask the opposite question:
Why do you feel that software necessarily needs to be modernized ?
I'm not sure how well SABRE works but I do know how fast and efficient keystroking through a non-GUI interface can be and I don't know why expert mode interfaces should ever be replaced by unsophisticated mousey-mouse-mouse ones.
At least, I feel confident in that analysis, since this exact same issue happened to Southwest in 2016, before they were using Sabre. Which would point to a chronic organizational failure
https://www.washingtonpost.com/news/dr-gridlock/wp/2016/07/2...
https://www.sabre.com/insights/releases/sabre-forges-10-year...
https://en.wikipedia.org/wiki/Sabre_(travel_reservation_syst...
It's not even an ASCII text app, but an EBCDIC one. Or was after EBCDIC was defined as a standard, after SABRE itself launched.