Spirit operates a fleet of recent vintage leased Airbus aircraft, so aircraft age and maintenance isn't an issue.
OTOH, Spirit also operates lots of one-off daily non-stops and they have no way of reaccommodating you in the event that a flight gets cancelled for any reason.
SW typically has enough flights from any station as well as enough connection opportunities that they can generally flow you through their system pretty close to your original schedule and this is true of most major carriers, although when rebook time comes, SW is very easy to deal with, unlike the legacies with whom you have no status.
These same legacy carriers also sell bare bones fares that subject you to the same restrictions as Spirit and Frontier including denying the use of the overhead bins or gate checking carryon bags.
Basically, this is like the soup Nazi episode of Seinfeld in that if you do everything right, you end up with the cheap flight you thought you were getting while if you screw up in even the smallest way, well, no soup for you.
Another possibility for those willing to spend a lot of time in transit is to look for bookable flights that involve multiple connections through indirect routings, very long layovers or red-eyes. These flights can often be booked quite cheaply if you use one of the third-party booking sites.
For the record, we don't fly Spirit, Frontier or Allegiant, nor do we spend twelve hours in transit on a thousand or two mile flight, but others of course do.