Would never work with Southwest's generous cancellation and no-show policies. If A19 decides not to show up, but doesn't bother deleting his BP, after A18 goes through, the machine would expect A19. A20 wouldn't be able to get in, because he'd be "cutting". It'd require manual override. Once manual override is enabled, the entire system is shot to hell and it'd be no different than it is now, because no GA is going to spend half their day shouting in the terminal "A19? Is A19 here??" any more than they're going to spend half their day telling A50 to go away when he's lined up with A20.