As I recall, those plans would have killed tidal movement in much of the bay south of the airport, turning it into a pond with little water movement. They deserved to be shelved in that case (and I am pro-airport, having worked there for nearly 40 years).
The expansion into the bay was envisioned when SFO's saw crazy growth rates during the dot-com days.
But subsequently the dot-com bubble burst, 9/11 happened, and United went into bankruptcy. SFO passenger traffic plunged, and the expansion plans were shelved.
Of course today SFO sees more traffic than ever before...