Any extension is all well and good. Going to Fort Mason sort of validates the use of the letter "F".
But whatever happened to the "E-Embarcadero" Line? It was to use part of the "F" line along Embarcadero, and then extend over the "T - Third" line to the CalTrain station and possibly beyond. The "E" would not require any new tracks to be built, just interlining with other routes.