I kind of have a different perspective. While I agree only children can certainly be well-adjusted, there is something special about a sibling. I was the only child for 10 years until my sister came along. I was happy. I loved hanging out with my grandparents and I got a lot of devotion. I still kind of felt something was missing though and was so happy when my sister came along. We were extremely close as she grew up, she was like a child to me almost. We didn't stay close really - we love each other, but have nothing really in common and I don't agree with a lot of the things she's done in her life, but I love her very much. She tends to take disapproval of her actions as a hatred of her, so we grew distant. I wish we would have remained close.
All I know is that I didn't know what I was missing until she came along. My children are 9 years apart and my oldest was very content as an only child - in fact, unlike me, he was very angry when he found out a sibling was coming. He wanted no part of it. Then he was born, and it was an instant change. These boys are so close and it is a bond like no other. My oldest was happy with other relatives and such around, but nothing like his brother. In their case, at least so far, it appears they will remain close, because they are two peas in a pod and do everything together despite the age difference.
I think in many instances there is truth to the "missing out" but then again - you can't really know if you didn't have the experience. Regardless, siblings or no, I think a child can indeed be well-adjusted and happy. It's no ones place to judge.