This will just ensure that WN continues to grow at the legacies expense. I just finished a weekend trip on WN and one thing that I noticed is that there is plenty of room in the overheads most likely because more people check bags because they are free.
+1. I had my first WN experience last week as well as a leg on Delta. I was shocked at how much faster the boarding and deplaning was on Southwest. Hardly a roller bag to be seen in the cabin. Delta took about double the time to load a slightly larger aircraft (737 vs MD90) and probably 3 times as long to deplane.
I have also recently flown Spirit, the majority of the customers have adapted to avoid the fees and turn times do not seem to reflect the decrease in hand luggage. Long term I can not see this being a positive change for American. First checked bag free for all on the other hand would differentiate them positively.