I think someone else said siblings are a blessing and a curse. And I agree. While siblings can be best friends, or good friends, a lot of bullying can occur within families as well. It is definitely not all happy families just because there is more than one child.
TBH if bullying etc is occurring between siblings then none of the children would be better off as only children. A parent who is unperceptive or chooses to stick their head in the sand when there is tension or antagonism between their children is going to be equally ignorant of any problems in an only child.
I'm so grateful that I have my siblings even if we did have some crazy fights when we were growing up. Like a previous poster said - there is something incredibly reassuring about knowing that there are people out there who have my back if ever I need them and like my sister said to me a few weeks ago - it's nice to be with people who 'get' you and don't need explanations.