This applet demonstrates the evolution of market areas. Points representing population sources (black dots) and other points representing shopping centers interact using a simple gravity model.
The basic notion is that a shopping center needs a sufficient customer base to survive. Customers shop at the most "attractive" shopping center, where "attractive" is a function of being close-by and having sufficient "size" (to have the desired goods). If centers have sufficient customer base (green dots), they grow and become more attractive. If they have too few customers (red dots) they shrink and lose attraction. If a shopping center shrinks too much, it will close. In this simulation, when a shopping center closes, a new center enters the market at a random location.
Even with these very simple rules one can experiment with how a retail landscape might evolve.