I ‘HEARD’ that the Southwest problems were due to not having any partnerships with other airlines for rescheduling. The system is called (IIRC) the Hub and Spokes policy. The other airlines were able to schedule some passengers on other airlines that they partner with. Southwest has no ‘partners’ so could not reschedule on other airlines.
Southwest changed from internal IT written systems to Amadeus many years ago for reservations.
Obviously not all of the internal systems changed ...
While WN does not look like a traditional hub and spoke routing system, that is what they have become.
BWI/DCA, DAL, DEN, MCO, MDW, PHX, BNA LGA ... probably several that I am missing. All of these airports have the slots and crew to manage 80+% of the daily load.
As to interlining, please note that it does take more than one airline to want to have an agreement.