You're talking about sustaining energy and calories for survival. I'm talking about physiological processes in your body (mood, brain function, hair skin and nails, etc) that SPECIFICALLY require amino acids in order to function (i.e. l-theanine for mood, l-glutamine for immune system, etc). Glycogen and fat will not help those processes. Therefore, your body will be forced to resort to stored aminos to keep those processes going, and even then, there are 8 "essential amino acids" that your body cannot synthesize from other aminos present in your body, so those will still be missing if your body doesn't get them from an outside source.
You want to talk about product marketing, though, your reference to casein protein lasting 6-7 hours.....now THERE'S product marketing. Yes, it's the slowest digesting form of protein out there, but nothing should take your body that long to digest and absorb. Not to mention what you really want is a variety of protein (whey, egg albumen (white), and casein) because each one has certain aminos that the other is missing, each one is higher in different aminos, and most importantly, they break down respectively in that order, so you get more of a time-released effect instead of whey digesting immediately, and then you're out of protein, or casein taking a while to break down, so you're out of protein while you wait.