“It’s better to shut it down elegantly rather than have it fail spectacularly”
I would think that an airline would base their system on Erlang, which is a language that can route around network problems and allows hot upgrades with no downtime. It was developed by Ericcson for cell phone networks.
“It was designed by Ericsson to support distributed, fault-tolerant, soft-real-time, non-stop applications. It supports hot swapping, so that code can be changed without stopping a system.”
https://en.wikipedia.org/wiki/Erlang_%28programming_language%29
Just because the language supports something doesn’t mean the software written in it does. One changing of a database table without somebody bothering to ask “what’s the new code going to think of the old data, and what’s old code going to think of the new data” and your hot swap just exploded in your face.