Thanks ATC_Watcher, and my apologies - I muddled SASS with what I meant to reference, which was ASSC (Airport Surface Surveillance Capability).
As I read the FAA's published info, ASSC works as I described, is designed to replace ASDE-X, and is already in use at Anchorage, Cincinnatti, Cleveland, Kansas City, New Orleans, Pittsburg, Portland and San Francisco. It is due to be installed at Joint Base Andrews next. Hence my point that an ADSB-based extra layer of cheese designed for large airports exists.
Yes, the LC made a mistake that he shouldn't have, that set off the whole chain of events. And yes, any one of the existing layers of cheese should have been enough to stop it. But it didn't.
I agree with others earlier in this thread, the fact that we aren't discussing a disaster is sheer luck that the FDX initiated a go-around when he did. A second later, and we would have had a collision, as indeed we might have if the SWA had rolled a few seconds earlier.