Chick-fil-A is very trendy and popular, and they don’t have enough restaurants to meet demand. The customers seem to be driving this, not being closed on Sunday.
If there were 2 equal restaurants, say 2 different McDonalds in similar markets, and one was open 6 days and another 7 days, would we see the Chick-fil-A effect? I’m betting not.
If this was the case, the Filet-O-Fish wouldn’t exist. It was developed by a McDonald’s owner in Ohio where the town was 85% Catholic, and didn’t eat red meat on Friday. He effectively only had a real business 6 days per week and was struggling. The fish sandwich was his answer and turned things around for him.