I can't speak for SouthWest, But I would not recommend this with many airlines. While the departure can change, many airlines keep the schedule for real-time check-in (as opposed to online check-in) to the original time. If you show up after the fact (or if you have checked luggage), you could be SOL.

As for aircraft substitutions, often when they "find" another aircraft, it is usually one that had arrived earlier that has already been prepped and can carry a similar number of passengers. They swap them out and voila, instant aircraft.

Generally, I will arrive for the normal scheduled time. I make sure I can work remotely just in case there is a delay and I need to do something. Hence I avoid any unpleasant surprises (such as a car accident occurring after I should have arrived). I never rely on delayed inbound aircraft as the situation can change at any time.

This. Two examples: I was once informed of a long (4+-hour) delay after I'd arrived at MCI for a flight to EWR, connecting to a flight to DEL. Those who left the airport ended up SOL; we were put on another plane going out "only" 2 hours late. And, when EWR was my home airport, I was flying out on a dark and stormy night and it was chaos. I pointed out to the gate agent that the arrival time of the inbound aircraft for my flight was impossibly close to the takeoff time for my destination and his response was something on the order of, "we're just figuring out what to do as planes come in". It was an honest "we don't know" answer and I really appreciated it.

One other thing I forgot to mention... At least for YYZ (and likely any Canadian airport with pre-clearance)... The CBP still close at their scheduled time. They don't care if your inbound is late... If you need to clear for a flight to the US, you best get to the airport when you should... Otherwise you will be taking another flight (and possibly be a no show if you have a connecting flight onwards).

Hub: I show up at original time. Even in the "my time is super valuable" case, modern technology allows me to work reasonably well from an airline lounge. In the "my time isn't quite *that* valuable" case, modern airline lounges allow me to drink reasonably well while I wait.

Outstation: I follow the tail and figure out what I need to do from there. Southwest will often post a delay early in the day but then the aircraft "catches up" if it proceeds through a couple hassle-free turns on its way to you.

In a broad sense, I have never been the "cut-it-as-close-as-possible" camp. I get wanting to minimize time in the airport. I am not going to show up 8 hours early for a flight. That said, I am not going to try to arrive 5 minutes before boarding either just to avoid sitting around either. I work with people like that, and most of them frequently miss flights or get on late, losing bin space and facing seat poachers. Time is one of those things it is better to have and not need than to need and not have.

There are ALWAYS unpredictable factors...traffic delays on the way to the airport being the topmost, but other things too..a little old lady infront you in the security line who is clueless etc etc. I make sure I am there at least 2 hours before. If nothing happens, I have no problem sitting in the airport. And if there is a delay, I still operate as if the flight is on time, because things can "undelay".