If I was a shareholder I would be asking why is the airline is happy to pay over 15 billion more fuel costs over the 8 years where there are other options that will reduce both fuel and maintenance costs.
Every airplane type has a myriad of fixed and variable costs to go along with it. Aircraft acquisition costs are a huge part of the equation, as well. While I would love to see more C-Series in American skies, if the total cost package doesn't equate then Southwest won't buy them. I asked a similar question or point some years ago and a very friendly A-netter was kind enough to detail a large number of the costs and expenses that would be incurred in adopting a new fleet type, including things I had never heard of nor thought about. Sometimes, buying the newest, most fuel efficient bird actually isn't the most economical response. Look at Delta and their fleet of older yet mostly paid for airplanes, and how long Northwest operated their DC-9s.