It's true that there are complications due to network effects. For example, improving a neighborhood can make it more desirable, leading to higher prices. If the development of condos also leads to the improvement of the neighborhood then that same effect could be triggered.
The knee jerk response is to block all new development. That solves nothing, as we all know.
The right response is to promote broad-based neighborhood improvement AND housing supply increases across the entire city and region. If only one neighborhood gets nice, then yes, there will be a lot of displacement. But if many neighborhoods get nice at the same time, then displacement can be minimized or even completely eliminated, because the stakes are so much lower.
I also think there's a place for inclusionary zoning or other affordable housing market interventions to soften the sharp edges of the market. Since no plan is perfect, and nothing ever works out as planned anyway, but we need to make sure people aren't harmed in the process, there's a need for tools like IZ, even though they aren't "ideal" free market solutions.
I think there's been a much better approach in recent years with the various CDCs and other non-profits building mixed market/middle-income/low-income housing alongside neighborhood or transit improvements in existing places. All transportation projects should be paired with a neighborhood building program such as that and more. I was really hoping that Somerville would be a leader in this respect, but now I am wondering about Curtatone... anyway, different thread.