Never is a long, long time. Circumstances can change in a way no one can foresee. It is possible that a great and permanent increase in fuel costs could make the buses and ferries much more costly to operate than today. Almost prohibitively costly. Same would apply to the SMART doodlebugs.

A Geary Street subway in the reasonably near future is a given, probably coming up at Franklin - that was always the plan. Park Rresidio is the cheap way, but a tunnel from the Presidio to Geary and Franklin would be a better alternative. A Marina route is always a possibility.

A series of really bad head-on collisions on the GG Bridge, even with a moveable barrier, could serve as the springboard for a campaign for a second deck. Remember this is a bridge, not the Mona Lisa. And it is a bridge that requires constant, expensive maintenance. If it is obsoleted it can be rebuilt. Public opinion can change practically overnight - just look at how the public has capitulated on offshore drilling.

Finally if you have ever sat on a Golden Gate commute bus stalled in traffic on 101 southbound
in Marin County, you will know why SMART will have negligible impact. If you look out of the window of this bus you will see that half of the vehicles are trucks carrying contractors, laborers, service people, deliverymen, etc. Marin doesn't do DIY; they are ritchies who contract everything out. Marin is just going to keep on adding more freeway lanes as long as Washington is willing to pay for it.