It seems like there are two main schools of thought against only children;
1. that the child will be damaged by incorrect parenting (spoiling, smothering, etc.)
2. that the child will be emotionally and mentally deprived because of the lack of a sibling
I wholeheartedly disagree with both. My husband and my best friend are both only children, generous, loving, responsible, normal, healthy adults. Who don't want siblings, and were happy to be only children.
First, there are many, many children who are spoiled and smothered who have siblings. Some parents pick favorites even amongst several children who they dote upon. Some parents overschedule all of their children, regardless of how many there are. Most "spoiling" comes from giving in to bad behavior, like responding to tantrums with bribery (candy, toys, etc). A good parent is going to be a good parent regardless of how many children they want- in fact, a parent is going to be the best parent they can be when they are happy with the size of their family. It's a really bad idea to have a second child you don't want because you're afraid you're going to do a bad job with the first. A better idea would be to speak to other parents, read books, or talk with a pediatrician or even a counselor if you feel your child is becoming a selfish or spoiled child. The most spoiled child I know is my sister. She is so spoiled, in fact, that she cannot even carry on an adult conversation at age 18- if someone offends her, she immediately begins crying or otehrwise drawing attention upon herself until the offender apologizes, and she refuses to forgive them anyways. Obviously she's not an only, I was four when she was born.
The second theory, that a child will be undoubtedly deprived of a playmate, best friend, and partner in parental caregiving later on in life, is also completely untrue. Of all the people you know, how many would say their sibling is their best friend? I know many people with siblings, and I cannot think of one who would say this, including myself. My sister is a friend, but not a best one, not even a great one. She has hurt me in ways that, had she not been related to me, I would have cut her out of my life long ago. Yes, we played as children. I also played with my friends, as did she. We also fought a lot, but no more than usual.
I have nannyed for a family with two little girls aged two years apart. Friends? Barely. In over a year of being with them every day, I could count on one hand the hours they spent playing happily together. Their personalities clashed terribly. They were always happier when alone. So there are no guarantees a second child will be a playmate for the first. Not to mention the possibility, however unlikey or difficult it is to imagine, that a second child could be handicapped mentally or physically to the point that play would be impossible. Not to mention even if they did get along, there is no guarantee the second sibling could get sick or have an accident and die, leaving a huge impact on the first.
As far as taking care of elderly parents, we would be lucky enough if ONE of our children took care of us, as in this society it is much more common to be locked away in a home. My grandmother has SIX children, yet once she became too mentally incompetent to live on her own (they decided), she was sent to live in a very nice home. Some of the siblings help take care of her. Some do nothing. My sister did nothing to help me care for our other grandmother these past three weeks when Nanna had a very difficult time recovering from surgery. And Nanna has a living daughter, who did nothing. Myself and one other granddaughter, out of a medium sized family, were the only ones to do anything helpful. So I would seriously reconsider that as a reason to bring another life into the world.
Sorry to write a novel. In my belief, there is only one reason to bring a child into the world, whether it's your first, second, or 9th-
Because you and your husband want to raise another child.