As a former (no longer in the Bay Area) user of GGT, and current user of a light rail line that replaced express buses, I submit that few SF bus commuters will volunteer to ride a train with city-transit seats and a bunch of stops to Larkspur and transfer to a ferryboat if their 1-seat ride on a commute express bus is still available. The bus might not be available because FTA doesn't allow competing federally-funded service in a rail corridor, but the N.Bay might be a little different because if I recall correctly the commute buses (not the basic service like the 80) are funded mostly if not all by fares and bridge tolls - so the FTA thing might not apply. And when the apparent ridership on the train looks like it's higher than expected, consider that the parallel basic lines will mostly likely be eliminated or severely cut back to force people to the train and to satisfy FTA.

On the plus side, as I have said about our light rail, the service isn't on the freeway so it isn't stuck in traffic as the buses often are, HOV lane or not. So while the ride may take longer (figure more like the 80 than the 72, because of stops and limited (60ish) top speed) it's consistent (though when things go wrong they tend to go very wrong). Since planned freeway and other road improvements in the N.Bay will have minimal long-term impact on congestion, that's a point in the train's favor. And if you want more train traffic how about adding a casino terminal?