I think the issue is that they have to make each lifestage playable with unique things to make it seem interesting. The problem with TS4 is it is very shallow. Most players play with Young Adult lifestage, then either move on to other sims, or an entirely different save by the time they age up to adult.
More early life stages are good if they can round out a child's development. Babies change a lot in such a short time, unless they cleverly write the coding for it, a single life stage can be anything between Crib bound infant, using a bottle, to a crawling almost toddler who eats babyfood at a highchair. And then switches back at the sims will. Do you know what I mean? It's a thing I'm worried the Sims 4 infant life stage will be.
Though, if they do the clever coding, and can give us progression with an infant, that maybe learns skills to be able to crawl and be weaned, that will be even cooler.