I'm an only child, and while I probably spent too much time with my nose in a book, I have never felt lonely. I was bullied in primary school for unrelated reasons, bit I had extended family and my parents were always doing fun things with me. I have honestly never wanted a sibling.
If I'm honest, I don't really see what the big fuss is about adult sibling relationships. Not that they are all bad, just that a lot I know have nothing really in common, live far apart and never see each other etc. I'm sure they love each other very much, but I don't look at their relationships and feel I'm missing out.
Then of course you have the brothers and sisters who are all fighting and bickering.
It just seems like a lottery really. If I could have been promised a sibling who I would love to bits, see as a friend etc, then yes I'm sure that would have been great. But it's just as likely that we wouldn't have had much in common.
DD will probably be an only child, I have secondary infertility and she's nine now, so even if we do have another, it will be during her teen years rather than her childhood
She does go through stages where she wants a sibling, but we've explained that while we would love to give her one, it's not happening for medical reasons that we can't help. She also wants a dog and a baby sloth too
meanwhile she has tons of friends, extended family including cousins she is close to, and she goes out to play every day with children in our street