While I chuckle reading your explanation I think you forgot to consider how much nursing is deeply ingrained in our nature by us being mammals.

> in a state of nature, they're likely to die anyways.

So while this part, where they would die if left alone, is certainly right, it would make the whole concept of us humans not laying eggs, putting some semen and mud on them and disappear into the wilderness an evolutional waste of time.

Plus, I have some acquaintances whom I suspect would very much prefer to go about their business that exact same way. So it's not a demand side problem either.