You are right, but it would disrupt a lot of cross bay transportation. If the west span went down, the waterway between YBI and SF would be closed. This means that the Oakland port, which is the Bay Area's biggest, would effectively be closed.
The BART transbay tube would be closed to check for structural damage from a massive object of the bridge's size collapsing.
Most Ferry traffic has to go under the bridge to get to the East Bay (Jack London Square, Alameda). I don't think that ferrys can go under the east span. The only place the ferry could go would be Vallejo.
Not to mention the thousands of people who live on Treasure Island.
Exactly. The Oakland port is one of the busiest in the world, and with the western span of the Bay Bridge down, it is essentially closed. That bridge is a single point of failure for the entire region (read shipping shut down, absolute unimaginable transportation gridloack, and the resulting economic effects) Not too mention the thousands of people on the bridge at any given time.