Except it's not. Autophagy as a process happens at the cellular level but the whole reason it's important is because it has emergent ramifications for the entire living organism, impacting on metabolism, senescence, carcinogenesis and a number of other endpoints that operate at organismal scale. Your characterisation of this as "fundamental biology" obscures this emergence.

Appropriateness aside, I think the real reason we don't have a Nobel for Biology is probably a result of the fact that Alfred Nobel cared more about the pursuit of knowledge for the betterment of the human condition, rather than for its own sake. I imagine he learned the limitations of the latter approach the hard way through personal experience considering his most (in)famous invention...

Why? This is clearly physiology (how living systems function). And physiology is a field of biology. Biology also includes things like the dynamics of ecosystems and classification of organisms.

Physiology is a more descriptive term for this work. You could argue this is "cell biology", but that's just a claim that these fields are exclusive of each other; much modern physiological work is now understanding the underlying molecular processes (maybe it is molecular biology?).

His work on molecular mechanisms has brought better understanding of higher level processes in the body and traditional "physiologic" mechanisms.