In a competitive industry saving money can mean lower fares for customers. It can also mean fewer total planes or engines produced reducing environmental
side effects of that activity.
There’s no need to have market based competition to keep costs down. Look at USPS for example. That operation is run without a profit motive, serves literally every zip code in the US (by definition), and runs healthily year after year (except for the part where Congress imposed arbitrary pension funding requirements[1] and private capital interests have been chomping at the bit to buy up all the real-estate holdings).
They also don’t have UPS’s problem of not putting AC in their delivery vehicles.
[1] https://ips-dc.org/how-congress-manufactured-a-postal-crisis...