Once upon a time I couldn't understand why companies and governments still used ancient computer systems running DOS, Win3.1, or 9x, or computers like C64s. "Upgrade! Your new systems will be far more powerful and efficient; and that means they're better!" -teenage me at some point, probably.

However, as I've gotten older I've realized that it's because "if it ain't broke, don't fix it". While Southwest may spend more money than necessary on maintenance due to the ancient systems needing now-specialized skills, those systems are also time-proven to be as functional and dependable as they need them to be. Ironically, they might actually be more secure than most modern systems due to a combination of decades of specialized security/stability patches they've probably had and simple security-through-obsolescence.

Edit: misremembered the phrase, "if it ain't broke, don't fix it".