[gnosis]

The gentrification that's happening in SF and that's described in the original article is happening because the rich are pushing out the original residents -- who were indeed poor.

From the article:

  Again and again, you hear of teachers, nurses, firefighters, police
  officers, artists, hotel and restaurant workers, and others with no
  stake in the new digital gold rush being squeezed out of the city.

  ...

  Bernal Heights in those years was a glorious urban mix of deeply
  rooted blue-collar families, underground artists, radical activists,
  and lesbian settlers

In general, the people described above would be considered poor.

As for high rise apartments not necessarily being ghettoes. That's obviously true. High priced apartment complexes and skyscrapers housing businesses are what constitute much of the "building up" in the cities you mention. It's only when building up is combined with low-cost housing that problems begin. But building up isn't the problem per se, but rather it's the high concentration of poverty that such projects encourage.

[greggman]

It seems like your missing the point. The reason there is no affordable housing is because the city doesn't allow building up so SF is stuck with only existing housing. As it gets more desirable those prices will continue to rise. There's only really a few possible paths forward

1) leave it as is. Prices continue to climb

2) allow building up. Prices come down as supply meets demand

3) some how make sf less desirable lowering demand