What happens is that town turns into Mexico. Or maybe worse than Mexico. The economy collapses. Corruption increases greatly. The town becomes broke, and there’s no money for anything. Infrastructure like roads, etc. starts to collapse. The water becomes unsafe to drink, and the sewage system starts to break down. Town government for all intents and purposes collapses.
We have a few towns like that around here, and they are almost like the municipality version of a failed state. The collapse is so bad that even a lot of the Mexicans can’t stand it, and they begin leaving in large numbers. It appears that at least this type of Mexican we have here in California needs at least a minimum number of Whites or similar racial/cultural types in town to keep the ship from sinking.