Wealthy people move in, raise property values, spur local economy, but displace lower-income families and the kinds of independent marginal businesses that served them. Or, wealthy people move out, property values plummet, services are slashed, land goes vacant, crime skyrockets, and kids can't safely get to school anymore.
What is the middle ground? I'm asking seriously. Imagine you were a city planner or the mayor of SF or Detroit or BDFL of California or whatever. What's your plan?