If you're continually adding plumbers at a constant rate, you will get an equilibrium. But the equilibrium you get will be different from the one you'll get if you had a one-time influx of plumbers and the market compensated for that. You'll get an equilibrium where the influx of plumbers continually drives the prices down and the compensating factors continually drive the price back up. Exactly where this equilibrium falls will depend on the relative rates of each part, and it may, in fact, be a net downwards effect.