Earlier quoted context omitted.
PG was wrong to say that you cannot be too strict about historic preservation because it leads to the exact problem we have in SF right now -- there is no physical reason this city can't be more dense and remain beautiful (look at Paris for an example of how this might be possible), but there are many, many political reasons.
There are also economic reasons. Parisian density is only possible with a great underground transportation system. That's hugely expensive to build and takes decades. SF has traditionally been a boomtown economy with corresponding bust cycles, so it's harder to make those kinds of investments. They are happening (e.g. the Chinatown extension) but more conservatively.
It takes decades as an artifact of governmental processes. There's no necessary reason it should take decades.