@uncanny_kate AerLingus, as an example, use a booking system they designed (and sold to other airlines) that was made in the 1960s.

It still works. It still saves them money. Other airlines have moved on to different systems. The cost to replace far outweighs the cost to update.

I only know of the system going down twice in 40 years. Last time was a back-hoe cutting fibre lines