The purely economic argument is not about which is the bigger sunk cost. It's not about which will create the most total economic value, either. It's about the delta in economic value from additional investment. College grads are already on an upward trajectory. Statistically and as a group, they're likely to create significant value without any additional investment. Babies, on the other hand, are entirely unrealized potential. Significant investment is still necessary.

In a way, your argument works against your conclusion. The more you've already spent, the less remains to be spent before the person takes off on their own trajectory. Fresh grads don't scale like startups do. They don't need ever-increasing investment to keep up with growth. They're (ideally) more like established companies, already capable of generating their own income and no longer in need of infusions from outside. They don't need our attention as much as babies do. That's why we call them incubators. ;)