That's part of the reason, but it doesn't explain how a customer could ever benefit from hidden city tickets on Southwest, which has no fortress hubs.
I'd *almost* bet the house that you can't benefit from hidden city on WN to the same extent that you can on the legacies. The order of magnitude is much, much less.
I don't know the intricacies of WN's pricing model, but if MCI-DAL was ever pricing at $497, there would be no point beyond DAL that I could book at $95. Perhaps an extreme example going all the way back to the OP, but the main reason is that for Southwest to have a $497 fare, it's probably due to real demand. (i.e., that flight is sold out except for AT/BS fares.) For DL to have a $497 fare to DTW, that plane could still be rather empty and selling $95 seats to GRR or whatever.
Southwest has fewer monopoly routes and less "weirdness" in their pricing model. Therefore fewer opportunities for a wildly lucrative repeat hidden-city strategy. But I'll concede a few could be out there somewhere...