I have a younger sibling, but she's six years younger. Honestly, I think I would have been fine as an only child, but when I was five, I asked my parents for a sister and they got me one. :)
My DH, on the other hand, is an only child, and probably would have enjoyed a younger sibling. There are all kinds of issues related to his childhood, though, so who knows if it would have been better or worse if another kid were around?
My advice would be to really try to ignore cultural and societal pressure or perceived pressure and the opinions of others and go with what you really want to do. Your kid will likely be fine as an only child, and you are likely very aware of the issues surrounding only children, and will work to counteract them if they become issues. And if you decide to have a second kid, it should be because you really want that second kid, and not merely to placate the world at large. Honestly, I feel bad for people who feel pressured to have a kid, have a baby, and then when the baby is about a year old, people start asking when they'll have their next kid. It just never ends. People reproductive choices are their own. I don't care if you have zero kids, one kid, or 10 kids, but do what's best for you and stick with it.