Even a captive audience will change their tastes, if their current tastes cease to be accommodated. They'll simply find somewhere else to go. But then everyone loses, because the restaurant is not given a chance to improve. The head manager in charge won't know WHY sales are slacking off. He isn't able to watch every diner experience, every hour of every shift every day they're open. All he is able to look at are the metrics. Unless he gets trackable customer feedback.