I have a two years younger brother, and my husband is a twin and has an older sister. I have given a lot of thought to this topic, because for various reasons, I think I only want to have one child, and whenever I say that, 99% of the time my friends will respond with "You can't do that! You have to give your child a sibling." Actually, I want to keep my sanity, I want to have alone time and time to read and maintain certain aspects of life that would be hard to do with several children (with one, parents can take solo childcare time and give their partner a break). I'm not going to do anything just because that's what you think I should do.
I think that, as with anything, any number of children you choose to have will have positives and negatives. But I think life expands to fill in for what's missing - for instance, my only family on this continent is my parents and brother. And I have no aunts or uncles or cousins at all period. So I have a very small family and never developed the close bonds others have with, say, their cousins. But I did instead have lots of friends and grew up into an adult who values her friend community as a chosen family. My husband has a huge family and really values it but doesn't have many friends - people work with what they have and develop differently based on what they need.
The common concerns that I heard being voiced about having one child are that they'll be spoiled, that they won't know how to share and interact with others, they'll miss out on a close relationship you get from siblings, that they'll be stuck caring for elderly parents alone. For the first few, you can actively work to expose your kid to others through playdates, group vacations with other friends/families, or outings where you allow your kid to bring a friend. If you don't spoil your child, they won't be spoiled. And kids who don't have close family or siblings can build amazing relationships with their friends, a whole community of potential future chosen family members.
My brother and I fought a ton growing up, and there were a couple years in our teens when we basically hated each other, but all in all it was great to have a sibling. He's a wonderful person, we both like each other as people, but I've also been living far from him for 8 years (ever since I left for college), and we're pretty different people, so he's not a daily or weekly presence in my life. That said, it's great to have someone else who has experienced all of our family dynamics and is also connected to our parents. My husband's grandfather died recently, and watching my MIL and her 3 siblings working together to take care of their mother (and their father at the end of his life) is really beautiful to see, and it's major that nobody has to deal with it on their own. Of course, it seems like in many cases there's one sibling (the more responsible one) who gets stuck with everything, and these situations are fraught.