Unless this place is very small, then you've missed out on some very important aspects of your scenario. Most towns don't have only one restaurant, which means that multiple restaurants will need to compete with each other. If 20 restaurants all compete for the business of race A, then they're each going to get a little less than 5% of the total population. A new restaurant would do better to gain a monopoly on that unserved population B than to compete for a small fraction of population A.
Furthermore, this hypothetical isn't really representative of anywhere in the US anymore. More realistically, the number of people who would refuse to eat at a restaurant that served racial minorities is itself going to be a minority, and likely smaller than the number of people who would refuse to eat at a restaurant that refused to serve racial minorities. So restaurants that cater to them have access to a smaller market, and those customers themselves have access to fewer restaurants, and would likely pay higher prices for less selection. In a realistic scenario in this country, those customers would be hurting themselves with their discrimination. What's more likely to happen now even in the absence of antidiscrimination laws is that almost all restaurants serve everybody, and the racists will end up eating at the same restaurants as the minorities even though they don't like
The owners who don't care about making more money will lose out to the owners who do.