On the flip side, I think having siblings can be heavily romanticised. Yes some people have good relationships with them but many people like me are completely indifferent to them or not close at all. And it's working in adult social work that made me comfortable with having an only child. More often that not, I see the care of elderly left to just one sibling anyway and I seem to come across endless cases of absolute horror stories of families falling out etc.
And I know many people, friends and cousins who are only children none of whom have any undesirable personality traits and are happy well adjusted adults. Of course I understand some people don't like being an only children but it's certainly not the same for everyone!