Well, being an only child myself; having a son who is an only child; and raising my son's daughter who is also an only child..., I can tell you, there are worse things than having an only child.
I wanted more children until I was about 37..., but I think it was more because I didn't have a choice on whether I could have more children (hysterectomy at age 28). Be that as it may, I was able to provide better for one child. We took our son everywhere we went and he was able to experience many things we would not have been able to afford had we had more children. He was articulate, intelligent and had an awesome sense of humor. He also had friends at school, so he wasn't all that lonely. He had one best friend named Ronnie, who was either always at our house, or my son was always at Ronnie's house. He has told me many times, that he actually liked being an only child. He didn't have to share his parents, and liked being able to go places.
Also, as was touched on by another person on this thread..., I adored my son. He was my life, and I'm not all that sure that I would have felt the same towards another child.
So, we are now raising a granddaughter. It's not as easy as it was when we were in our 20's or 30's and 50 is starting to feel old. She is extremely bright, very articulate, we take her everywhere we go, and she's been to Hawaii three times (she wants to move there).
So, I guess my point is..., you are the only one who can decide whether you want another child, and decide how old you want to be and still be raising children. My advice would be, only have another child if it's something YOU want or need to do. Please don't do it for your daughter, your husband, or anyone else.