Everyone here is at least a little misinformed regarding their software and it's simulation abilities. They have control systems with the data they need, and this data is fed into the flight scheduling & monitoring UI. Their tech works, it just gets bashed on because it's different than a lot of other airlines. Used properly it can account for these things.
What causes things like this is overbooking. Overbooking happens in more than just butts in seats. If you're not going to have enough ground crew to handle your SLA on flights, flights should be preemptively cancelled to keep them under that SLA. Airlines keep that SLA as low as safety permits. The max is set by the FAA at 3 hours for domestic flights and 4 hours for international flights (both ends).
Southwest's software is doing it's job, it has adjustable tolerances. It will even take into account weather conditions reducing ground crew therefore raising SLA. But, the effect of the weather conditions on the ground crew is also adjustable by humans. As a matter of fact, while one would think it would just reference historical data, that's not exactly true. It references predictive models that are adjusted by experienced individuals. Those individuals can be ordered to adjust the parameters outside their honest assessment to allow for steps at the beginning of the process to operate smoothly.
Weather prediction has a horizon. Southwest allowed excess bookings beyond that horizon, or they allowed excess last minute bookings. This weather event was massive and one-sided, yes, but it was also completely predictable. A human made the decision to widen the guardrails. There aren't a ton of people allowed to do that, I can think of maybe 3.
Disclosure: I wrote software for SWA many years ago. Nothing I've said here is privileged, in fact most airlines operate this exact way.