Popcorn has been the main source of food in cinemas since the birth of cinemas themselves(don't quote me on that). They're a comfort food and they should continue to front line the main snack people buy when watching a movie. 

However, the menus in cinemas can be spruced up and I believe introducing a bucket/ box of french fries (as shown in the picture below) can potentially drive your business up.

There's nothing else to say. I'll leave you with a quote I came up with:

"Fries is life, fries is love, fries is always one step above".