I'm an only child, my DM had two miscarriages and a stillbirth in the years after I was born.
My DH is an only child, his parents seemingly had a child as it was 'expected' of them. They didn't care about him as a child, and even less when he grew up.
Our DS is an only child, by choice, as his was a very difficult birth and I was really ill afterwards. We decided that another birth wasn't worth the risks involved, especially given my DM's obstetric history.
Our DiL is the eldest of four girls, and was used by her parents to basically 'raise' the younger three. As soon as she was old enough, her parents would vanish off abroad for holidays, leaving her at home in charge of her younger siblings. This was not on some 'sink' estate either, very well heeled, educated parents, they just didn't give much of a damn.
Our DGS is also an only child, by choice. DiL would probably have liked more children, but we almost lost both her and him at his birth, so there's no way that another birth would be risked.
My DH, DS and I are all perfectly happy being only children, and so far, our DGS, he's almost six, is fine about being the only one.
I'm sure that there are some very lonely and miserable only children out there, in fact my DH was one until we met. However, even if he had been one of many, his parents still wouldn't have had any interest in any of them. They were just very insular, selfish people.
I've had a wonderful life as an only child, although I suppose there have been a few moments when a sibling may have been good.
When DF died at the age of just 61, and I seemed to become responsible for DM's happiness from then on. She just wasn't interested in anyone or anything apart from me, for 21 years, and it wasn't always easy. My DH was an absolute wonder, but as his own parents were so cold and unfeeling, he'd more or less 'adopted' my DM as his own.
When DM was receiving end of life care and I had to make all the decisions by myself, that was a tough time. Perhaps then a sibling may have been nice, but hey, I grew up, as do a lot of only children, to be self reliant and strong, so it was fine really.
If I had my time over, would I choose to have had siblings?
No, I would not!