FAs on domestic USA flights aren't going to run back and forth from the galley serving snacks and beverages when someone randomly makes an order. At best you could pre-order, but they would still bring the orders down the aisle in a cart. Lots of people wouldn't have ordered, so they would just make an order then and there. So it would end up being pretty much like service is today, except maybe they wouldn't run out of snack boxes.