I was an only for years, until my mother remarried (a man with kids), then had another child.
As an only I was very self-sufficient, happy to play by myself, had a very happy time, lots of interests and so on. I also would go over to friends and have them to me, so I spent lots of time with other kids.
I was an only child until age 8 and I remember it as a good time, and I retain many of the interests that I had then (astronomy, voracious reading, swimming and so on). I never, ever felt the lack of a sibling and had plenty of friends to hang out with when I wanted..
My experience of being a sibling has been largely negative. My step-sister was and is a nasty, lying bitch. My step-brother has been great during my recent troubles (for which I am very grateful) and we have a good relationship, but tbh, if we weren't siblings we wouldn't spend time together. My little brother and I have a good laugh when we're together, but we don't really keep in touch.
So all in all, siblings haven't really improved my life on balance, and being an only was fine.
The only doubt I have about my own - only - child is that he'll be entirely alone when I die - something that I'm no longer looking at in my own case. I imagine it would be quite bleak, and would like to hear from other adult onlies who either have lost their parents, or have views on how that will be.