I just upgraded at my shop, and after 7.5 years as a FO, I have no desire to spend the better part of 2 decades waiting to upgrade.
I can handle the 10+ years at United, AA, Delta, if there's some sort of light at the end of the tunnel. Combine that with the ability to be a FO in multiple types, and a retirement not requiring a match, I can see why Southwest has problems. The only way I'd go there is if they offered a signing bonus, and then I'd still try to leave.
Delta's not even ten years now I don't think.