Both my parents were only children, which meant I had no aunts, uncles or cousins, but I don't think I've been psychologically scarred by that!
My mum says she didn't mind being an 'only' when she was a child as she had plenty of extended family. But she found the burden of caring for elderly parents (at the same time as she was herself raising a family of four) an awesome and terrible task. Now she herself is elderly she feels very alone, with no one to talk to about 'the olden days'.
I don't think that's a reason to have another baby, though. It's just something to think about for the future, such as planning adequate care in your old age (who doesn't have to do that, nowadays???)and keeping lots of memorabilia, labelling photographs, noting special events, milestones etc so that your grown child can access things they may have forgotten about themselves.
FWIW, my adult niece is an only child and I know lots of other people, both adult and child, who are only children and they all seem as 'normal' as the next person, to me. The concept of the spoilt brat is not the sole province of the only child, either!
Good luck in your decision, just do what feels right for you and your family.