i don't know if the existing residents will want to stay, but i think many more people overall would want to live in a manhattan of the west coast, as evidenced by the manhattan of the east coast which people keep moving to.
So go build "Manhattan of the West Coast" somewhere else on the coast, and leave San Francisco alone.
San Francisco ain't my backyard, by a long shot.
> some big city along the northern California or southern Oregon coast would make the corridor from Vancouver to San Francisco more contiguous, which could help to justify a high-speed rail line.
AFAIK, the geography of the Northen California to Southern Oregon coast isn't real conducive either to a high-speed rail line or a big city.
(If you want a Manhattan of the West Coast, it probably would make more sense to try to build it in the LA area, where an extra couple million people packed in a tight area would still obviously require radical new local infrastructure where it came together, but wouldn't be as significant of a relative increase in demand on regional infrastructure.)
Given that the probability of a new big city on the West Coast is effectively zero, I felt comfortable ignoring real limitations of that plan. In reality, of course, water, seismicity, and erosion put a damper on any attractive hypothetical options.
It just doesn't make sense to transform a city to something its residents don't want because someone else has an aesthetic preference for it to exist somewhere.