Gee, it’s almost like offering absolute shit service, making everything an additional fee, constantly canceling flights, and never being on-time is a poor business model. Who knew?
That isn't the reason the ultra-low cost airlines (like Spirit) are struggling. Its the opposite. The bigger air carriers simply copied the Spirit business model. Check out United's Basic Economy fare. Its the same "cheap seat, but nickel and dime for every upgrade" that Spirit does.
If you get all the extras it many times bumps you higher than a normal ticket from the larger airlines. As long as you know that, and are willing to fly very light and basic, it can be very cheap to fly the ultra-low cost airlines (like Spirit or Frontier).
In short, if you want full service flight, don't go ultra-low cost.