You'll notice that neither San Mateo nor Santa Clara Counties want to "join BART" even now. They're interested in, and have had to find money for, specific extensions. But there's no "Ring the Bay" movement except maybe in some back rooms where funny things get smoked or Good Stuff is available for drinking. Caltrain will be around for a long time if they can find a reliable funding source, connecting with BART in a couple of spots. The BART District is still just SF+CC+Alameda Counties.

Marin was supposed to be part of the original system too, but was always iffy, and when it became clear that running it over the GGBridge wouldn't work (without essentially building a new bridge) they were happy to drop out. Sonoma-Napa-Solano were never really in the picture, though there was some talk of extending BART to Fairfield and someday Sacramento that never went anywhere -- which makes it kind of ironic that Capitol Corridor trains are now under BART oversight. So "Ring the Bay" was always more like "Ring the South Bay" even before the dropouts.