As a passenger, another factor is SWA's service culture. While I'm not a fan of open seating, once you get "past" that- emotionally, mentally, and physically- their service shines. Of course, that is considered in a coach environment.

On legacy carriers the availbility of upgrades has become much lower. The quality of first class (at least domestically) is really not good. I've had multiple instances of getting upgraded domestically on legacy airlines where I didn't "deserve" an upgrade (so to speak) and, after the flight, thought "why in the hell would anyone be loyal to any specific airline for this?"

But it's not only the "lure" of first that has declined- service is subpar in coach as well.

Have the legacy lines diluted the benefit of frequent flyer programs? Yes. They've made them a profit center- through affinity programs, etc. So, in effect, they have strayed from their core product if you will. I don't know if that is good financial decision because I don't know the actual financials- but, right now, it's a fact (IMO-not humble-but experienced) that SWA is kicking butt on ticketing, gate, and cabin service as well.