I think it's impossible to free babies. They are literally objects so they can never be a sim.
It's only so much you can do with an object. An infant is very much a baby. It seems the infants we got are not even a year old yet and can not even roll over without learning. I think the way they did it opened up so much more opportunities to play with babies and watch them grow. If they just gave up a newborn sim there would be nothing to do with them. Nothing special like teaching them to roll over. Watching them crawl and giving them their own little quirks and personalities. Because newborns just eat sleep and go to the bathroom ?