My first job was in a Little Chef. I did about 3 years in one during & after college as a griddle chef.
The great thing that LC had going for it in those days was that it wasn't fast food in the McD's sense but it was food that could be cooked freshly in a short space of time, i.e. short order food.
The best sellers were always the breakfasts, the fish & chips, the burgers and the salads in the summer. The beauty of all of these was that they were easy & quick to cook from scratch. They were tasty, the ingredients were inexpensive & relativley good quality and back in the day ther represented reasonable value for money.
If you've got a menu full of smoked salmon & poached eggs, fresh hotpot and panfried fish mixed in with your Early Starter and Magnificent 7 Burger. The quality if the ingredients is goint to suffer badly, you can't get decent cuts of smoked salmon cheaply and you'll end up with loads of wastage. You'd have to refit all the kitchens to cater for things that can't be cooked on a griddle or in a fryer. And the time it take to prepare a meal will double at the very least.
At times there were 18 or more orders following what I was sending out, with maybe 4 meals on each order. Two people could clear that in 30 mins easily. You couldn't do that if you have to ponce about with pans of water and a complex menu.