Back Bay of Boston is really a marvel. I have literally no idea about California so I'll have to take your word for it. My concern is that the only people who seen willing to build are those who stand to make the most money. It would seem that the effective way to do this would be to require as part of codes for building different cost zoning to be taken into account. When I was in Tokyo they had elevators sectioned off by floor access, which would provide the ability to have luxury, shopping, and lower/mid income mixed into the same building. When I've traveled to Europe I've been interested in how businesses have been lower levels with hosting above, something I've not seen much of in the US.