We have an only child by choice, she is 17.
You'll hear about the stigmas of only children and it's BS.
I've seen plenty of spoiled brats who are not only children. It is about parenting and teaching your child. Just because someone is an only child does not mean the grow up with no one. That's a silly thought... Think about school, social, friends, neighbors, etc
As for later on, our daughter is close with extended family, we've provided for her very well for her education and more. We've also provided our own set up for retirement and she will have no worries as far as us being a burden to her as we age.
Some people have children to ensure their retirement and that's not why we chose to bring her into thus world. There are no guarantees that siblings will be close and have good relationships as well.
You do what is best for your family and don't worry about naysayers. My house has always been full of children because many have told me that they prefer to be here vs at home arguing with siblings. etc.
We have no regrets. I know my time is now limited because she'll go to the college of her choice in the fall of 2013. We cherish our stable, secure home.
Good luck to you.