As an only child, I would urge the couple to think of the blessing at least one sibling could be to their child. They will have a shared history of their parents always, and of each other, an ally instead of feeling outnumbered by the adults. They will have an easier time interacting with other children due to being socialized with another child in the home. When the parents are elderly, they will have less risk of having to bear the care burden all alone.
It’s common for people, I suppose, to want to frame in their mind an “ideal” family size - I always had hoped I could have either 4 or 6 children. But I think the concept of attempting to deliberately choose to have an only child should be discouraged, for the reasons listed above. :twocents: