That is really interesting. I am surprised by the number of people who
do accept a similar offer.
(Also, I am surprised at the number of reports where the voucher didn't get sent.

Also, one particular example where it turned out the voucher was only good for 30 days! Hopefully, that wouldn't be the case on SWA)
I guess if you were considering making a change
anyway, it could be a win-win.
I wonder how successful SWA is going to be in making these offers in places where they have 1 daily non-stop? (Or, 1 weekly non stop on the weekend...)
What if the new flight is already priced at a lower fare than what you paid? (Probably unlikely, but I guess you would want to check anyway)
Finally--I'm just getting used to the idea of seat assignments--but, in the future that could have to be a consideration. Would you move to a flight that didn't have your preferred seat options for such a nominal amount?
One of the theories posted in the thread above was the purpose of making these offers was even though the flight was not sold out--they wanted to free up space so they could resell the seat to a last minute customer at a higher price. Maybe some of that high level AI thinking?