I am one of two, but do not get along with my sibling at all. I am the parent of two girls who get on very well with each other. I cannot pretend I have any experience of the only child upbringing. I do however have a few only child friends in their 50s who seem perfectly content with their lot.
I am posting just to make a comment in my experience of being a consultant psychiatrist for nearly 30 years. I have heard every possible sad story of abuse, trauma and hardship. When I take a history, I have to ask everybody about their childhood.
On quite a few occasions, I have heard about specific problems with siblings which have contributed directly to poor mental health, and also issues with growing up with parents who could not handle the number of kids at home. I can hand on my heart promise when I say that nobody has ever said to me that they have been unhappy because of being an only child.
I accept that it happens of course, as some posters here have indicated. But I think it says something that when patients talk to me about adverse effects in their life, they have never talked about the only child experience. And I have seen hundreds and hundreds of patients over the last few decades. So statistically the chance of serious unhappiness about being an only child is pretty low. (But when it happens, patients deserve to be heard of course and the pain is real).
I just thought it might be worth sharing my experience as a mental health professional OP. There is so much more to a full and content existence than the number of siblings someone has.