I don't disagree with your post at all but I will point out 1 fleet is a heck of a risk to run after the FAA generated AD debacle on the 80 series that AMR and DAL endured in 07.

Say the 737 has a design flaw that grounds them all. We lose 11% of our fleet or 14% of our domestic fleet (using 1am math mind you). SWA would lose 100%.

They also mentioned during the merger that the cost efficiencies found in a single fleet when it comes to maintenance and so forth is already maxed out given the large size of the existing MD, Airbus and Boeing fleets.

However, there are still plenty of inefficiencies including pilot training and cost.

But I agree with your post.