tbf it's debatable if there 's a lot to learn from C.elegans. Simple animals have been studied for decades from aplysia to the mouse. But those are not behaviours that are interesting when attempting to learn more about the human brain. The Allen institute's connectome project is more relevant to mammals, even if it's only a tiny volume of the mouse cortex, in order to mildly constrain models of brain function. Even if we had the whole brain, it s too large to be simulatable. These data help our understanding, and we 're lucky we have amazing tools to probe brains at this moment. But we need more and better theories to put them to good use
I don't even know where to begin rebutting your argument. Most of the stuff we actually know, actionable knowledge that has withstood the test of time, comes from small animals, including C. elegans, Drosophila, Aplysia and others. This is because a lot of the stuff is genetically conserved. Even things that are considered specific to mammals, neuromodulatory systems such as dopamine, serotonin are highly conserved. For example worms, flies etc they all get hooked on cocaine through what's thought to be a very similar pathway. Pathways governing such "complex" behaviors as learning, memory, exploration, exploitation etc. all seem highly conserved, which means that a lot can he learned. Source: I'm a worm neuroscientist working on mathematical aspects of neuromodulation of behavior.
c elegans has very primitive, simple behaviours. It's not really possible to get something useful out of it about either our cognitive functions or our brain disorders. The things that regard single cell pathologies (e.g. plasticity) are already studied in vitro in mammalian cells. There s probably many cognitive phenomena that only become apparent in large brain sizes, so i m not sure this method scales up.
In what situation WOULD you be able to expect to extract "something useful" about "our cognitive fucntions or our brain disorders"? It seems silly to think we could learn anything about such complex things without understanding something simpler first, hence the approach of validating models of simpler structures.