The only upside will (briefly) be not having to deal with the TSA and showing up ~2 hours before the flight. Unfortunately, the TSA is making an appearance in local metro and train systems more and more, so I expect them to show up pretty quickly.
Then we'll have an grossly underestimated project with higher than "expected" costs with high ticket prices and no tangible benefits over the alternative - and likely faster - mode of travel.
And odds are, the system upgrades will need to start immediately so the ongoing maintenance will not be significantly cheaper than construction.
The Transbay Terminal is in a lot more convenient location than SFO. Lyft to-and-from the airport adds $60 to your fare.
Also, I don't know what the price model will be. In Italy, for instance, you buy a ticket reflecting the route you want to take and you can get on any train you want. Miss the 4:05? Take the 4:25. I wouldn't be surprised if this was similarly flexible. Most airlines, on the other hand, charge change fees unless you are a frequent flyer.
