All-you-can-eat restaurants should not exist.
People with large appetites crowd into all-you-can-eat establishments. These greedy customers eat vast amounts, driving up the restaurants' costs. Restaurants have no choice but to increase prices, but this turns off people with small appetites. Eventually the only people who eat at the restaurant are hulking athletes, and the restaurant fails - a victim of "adverse selection".
All-you-can-eat restaurants also suffer from moral hazard. Patrons can fill their plates for free, so eat until the marginal benefit of piling on additional food is zero. It is trivial to show diagrammatically that all-you-can-eat pricing is inferior to marginal cost pricing: