Abortion in cases where the baby is certain to have heavy disabilities is completely ok and morally justified.
By letting that child live you subject yourself to a life of misery. Your sole purpose will be caring for that child 24/7. It will never have children of it's own. You will spend all your money on the disabled child. And if you have any other kids you will subject them to a miserable childhood because you won't be able to spend time with them as much as they need. They will grow up poor and overlooked because the disabled kid will have all your attention. They will grow up poor and emotionally stunted.
The disabled child will be nothing but a burden to everyone around them.