Agree. It would be tough to do that. But they could say assigned seats are guaranteed only if you get to gate area 20 min prior to departure. SWA does not often start boarding before then. Sort of like an A BP; you are guaranteed to board before B and C only if you are there before they board.

Also, I find few people in Gate Area just before departure -- as one cannot predict security lines. But you are right, with legacies you can be last to board and still get your seat (but may not have overhead space). In fact, most like to board early.... otherwise, why do so many First Class board when they start?