Market forces will definitely apply, but my concern is that the market will react too slowly. That by the time the market can react and raise wages the labor pool will be shrunk too much and either services won't be available, or they will be too expensive for even those who could otherwise afford to live in the city. You can't just get a labor force to move back to an area overnight (just like these problems aren't going to appear overnight). So, in order to compensate, the market will swing wildly until it can stabilize.

That's what I'm worried about. Populations shift, people move, cities grow. But, if you're running a city, you want growth to be stable and predictable. If you drive out all of your lower-income workers to other cities and "import" your labor, you are setting yourself up for an unstable labor market in the future.

These things need to be planned for, and it doesn't seem like there is a lot of thought being put into it.