Original poster here, circling back. I've flown Southwest a few times last year and didn't buy a second seat. I actually exploited their policies to ensure that if there is even one empty seat that it would be the one next to me.

So, this was during the open seating phase in 2025, but after they ended the free checked bags. Their fares happened to be lower than other carriers last year (2025) so I flew Southwest quite a bit. At the time, their basic fare pretty much had no restrictions other than earning less points. Their basic fare with the early bird checkin addon would often still be less than the regular (non-basic) fare on another airline. So, I would book a basic ticket (I don't fly Southwest enough to care about the Rapid Rewards points although I do still accrue them in case I accrue enough to eventually redeem them for something) and then pay for early bird check-in. On top of not having to set an alarm 24 hours out to go check in manually, I'd get an A position, and when I boarded, depending on how full the flight was, I would often pick the window seat in the row in front of the exit row. That row is less desirable since it doesn't recline. As other people are boarding, I'd make eye contact with anyone who appeared to be a customer of size traveling alone, and they'd understand to take the aisle seat. So, even if the flight went out with 1 seat empty, I guaranteed that the middle seat next to me was the one that would be empty. On flights that were wide open, I'd sit in one of the first 5 rows since they have an extra inch of legroom.

Now, I haven't flown Southwest since the switch to assigned seating, but if I do, I will definitely book two seats (Yes, I am aware that I'd need to book a Choice fare now to make this work)