We have an only child. We'd have liked a second but it just didn't happen and we weren't "that" bothered to go through medical checks, IVF etc.
It's been absolutely fine. We've compensated by making sure he did lots of hobbies and activities where he met other people outside school. He benefitted massively by lots more "quality" time with us not having to share our time and attention. He's got a big circle of friends, both locally to home, in his Uni town, and online.
We've asked him many times over the years if he regretted not having a sibling and he always said no, what you've never had you don't miss, etc.
I think we were worried because both of our parents were "only ones" and constantly whinged about it and they both ensured neither me nor my OH were only ones, but neither of us have strong bonds with our relative siblings and are both pretty much non/low contact with them. I think it's different times now with the internet, social media, etc. My own parents were old when they had me and their childhoods was in the 40s and 50s so not even telephones, television, etc and certainly no radio, so I can understand how they felt more lonely and isolated without siblings, but they also had no extended family living nearby either so for both it was literally husband, wife and child, no uncles/aunts/cousins etc living nearby and no forms of contact other than school. As I say, different times now, and I do think it's one benefit of social media to bring people together who can't be together in person.