Cool, that's what we've been lacking, more Bay Area regional planning talk!
I wasn't aware that BART ever included plans for a loop- I though it always was conceived as a commuter feed system from the outlaying regions into SF, with the big holdout being Marin.
I think BART was sort of doomed from the outset as any kind of traffic alleviator, since urban planners in the 60s always assumed that San Francisco was the hub. The growth of the Bay Area as a megapolis requiring a distributed, regional transit network (mesh instead of hub) didn't take off until BART was pretty much cast in stone. As it stands, there are too few tracks and too many choke points to be particularly resilient or to be able to significantly increase capacity.
The best to be done now is extend lines into the areas of outlying growth, as in the relatively recent Pittsburg/Bay Point or Dublin/Pleasanton lines, and of course south down the Peninsula. But in the new cash strapped Bay Area (or US, for that matter) giant infrastructure projects seem like a dim hope. I drive the Bay Bridge every day and look at the modest thing that's going to be the new Eastern span and shake my head at how far we've fallen since we made the Golden Gate.
I think another test of success is warranted. When BART was first proposed, while it was under construction, and even for some time after it started running, it was frequently criticized as completely unworkable. Today, I would dare anyone to consider getting around the Bay Area without BART in the mix, whether they are riding the system or driving a car. Many American cities would be grateful to have such a doomed system.