[lolpep8]

Old buildings have to be demolished and new ones built, which may not be in the range of the former tenants. Old residents have to leave either way, which is both the problem itself and the root of other ones.

Anyway, it's the mutation of a city into another one, there are too many variables. Higher density sounds appealing but it may bring other problems, and the old residents end up displaced anyway. It's not going to be pretty, and the result is not going to be the San Francisco everyone loves, but well... it's not my city so why do I care.

Edit: quite a few edits

[yummyfajitas]

If you can build arbitrary amounts of new housing the only way that anyone will be displaced is if (marginal cost of building new unit, amortized to a monthly rent) > (what old tenant can pay).

You are right that new Pune isn't the same Pune it used to be (or so I'm told) - now there are Gujus, Punjabis, Africans, white dudes and other non-Marathis there. As an American I'm a bit unsympathetic to "there goes the neighborhood".

[lolpep8]

Buildings take time to build, do I have to make that clear?

Also, it's not about ethnic make up, it's about culture and identity. The neighborhood can go in a wildly different direction, and that's what people are lamenting, specially if it's culture-producing centers (let's admit it, the most that tech nerds produce are open source libraries for other tech nerds to consume). Anyway, urban centers come and go, SF's death will be a sad thing, but it happens all the time.

[Gurkenmaster]

>Buildings take time to build, do I have to make that clear?

That largely depends on how quickly you want to build. Chinese companies can build whole skycrapers within weeks. http://www.theverge.com/2015/5/14/8608039/building-timelapse...