I guess the end was supposed to be terrifying, but really, without the computer complex etc, it's really just a baby. It can't kill people by remote control any more. And it doesn't have all the information gathering, camera monitoring, and so forth capabilities that the computer did.
So what is there really to be afraid of? If it says "give me another woman to impregnate" you say "No" and make it sit in the corner or send it to its room without supper.
The child still has the intelligence of Proteus, and (hopefully) a civilised society will listen to the child.
It had a much accelerated growth rate. I assume this would continue outside of the incubator since the body would grow too big for it. It has the intelligence of Proteus, as stated by [deleted] and had already come up with a cure for one form of cancer, so imagine what kind of "worship" it could get if it offered to do more, giving it a form of control over us. It also created that amazing and kinda-spooky pyramid-like snake machine, so imagine what kind of engineering abilities it had.
The other thing is that the couple were given a new child after the loss of their first child. They'd have to kill it right there before anyone else knew about it, if they could bring themselves to do it.