That’s a common misunderstanding, restaurants are low margin due to competition. As long as the industry moves in lockstep with cost increases the margins stay the same.
Sales taxes for example vary widely yet have minimal impact on low margin business. Lower them and customers save money but restaurants margins stay the same.
For the same reason though, complementary goods don't necessarily have the same tax burden or margins. So the distribution of purchases might shift. Restaurant-goers might just eat at home more, or maybe even just go out to more expensive places that can operate with lower margins.
The important bit is the number of restaurants may change, but their margins don’t. The classic Starbucks at all four corners of an intersection is predicted on the total number of customers in that area. Cut them by 25% and 1 of those 4 locations but close 1 location and the the three remaining have identical numbers of customers as before the change. Cut by 75% and your down to one location, but again it ends up seeing the same number of customers before and after the transition.
Sales taxes for example vary widely yet have minimal impact on low margin business. Lower them and customers save money but restaurants margins stay the same.