Had this exact discussion with Mike V and Chuck M a few months ago.
The short answer was, Southwest will never charge for bags until the money offset from customers choosing SWA so they don't have to pay those fees is SUBSTANTIALLY less than what they could make charging for bags.
Right now, it's estimated they make a lot more money with their "Bags Fly Free" policy (and the associated ad campaign bringing in people who have been burnt by bag fees, especially with Spirit and Allegiant) than they would if they charged for bags in their system.
There's also other ancillary fees that help make up for it. People paying the up-charge to get the first boarding passes, etc. Their response was more detailed than that and had some proprietary $$$ info to it that I can't put on here, but the impression that I got was that they review this topic on a regular basis, have detailed estimates on the exact dollar figures each way, and they believe they aren't even close on reaching a changing point yet.
They're certainly smarter than I am about a lot of things, including revenue management, so I'll just keep flyin' 'em.