I take sick pleasure in reading posts like this. They keep alive a dream that will almost certainly be dashed by the intellectual giants at the SFCTA.
One point that The Overhead Wire touched on that I think is important is that a proper setup for this corridor involves grade separated rapid transit (subway) in addition to an improved surface line.The street in SF most like Geary is Mission Street. Pretty much along that whole corridor (though it deviates by a handful of blocks outside the Miracle Mile) you have BART running under the 14/49 complex.
The two complimentary transit lines are not redundant, they provide for a diversity of transit trips and increased ridership overall.
Geary needs rapid transit to get you from the dunes to downtown, and it needs attractive local transit to (a) get you to the more distant rapid stations, and (b) take you from your home to the neighborhood amenities.
If the SFCTA has such a burning desire for buses, let them continue to run the 38 on the surface, just as Mission has the 14/49. But a surface line is not going to solve the demand for rapid transit. The money being spent studying and building BRT on Geary is being wasted in the name of saving it. The expectations they're setting - that BRT is like rail for the cost of paint - will not be met, and people will distrust Muni even more.
Whether BART or Muni builds the tunnel under Geary is relatively unimportant to me. But it has been clear since before they ripped out the streetcar tracks that a tunnel is what we need. And for the cost of building it, it would be far more well used than any current extensions.