Yes, the current tube handles all but one of the lines; but so does all the other track between Embarcadero and Daly City. All of that track still has to undergo maintenance as well as all the track to Richmond, Pittsburg, Dublin, and Fremont. When would all those tracks undergo maintenance? Would they single-track all the way to Fremont? They could do that now.

All a second tube does is enable BART to run 24 service across the bay between (say) the financial district to Oakland (probably from different stations). It’s not clear what you do once you get there to go the rest the way to your destination.