Among the reasons WildCowboy is correct about the HSR is that bringing CalTrain to the TransBay is not under the purview of the HSR authorities and the city is determined it will come into the new terminal. Once the tracks and tunneling is done, it would be silly not to bring at least some HSR service (one proposal has some HSR trains coming into the TransBay, some terminating at 4th & Townsend) into the new terminal.
As for the terminal itself, not only is the location and design pretty firm now but the money to build the terminal building (so-called "Phase I") is in hand. So it seems as certain as anything can be that it will be built. When/if the tower and the CalTrain tunnel from 4th & King (so-called "Phase II") follow is less certain.