I think this is one of those things that so, so depends on context and your family situation. I am an only child and I loved it and it was absolute best choice for my family situation. My parents marriage was rocky pretty much from when I was born (my dad really didn't like being a parent and wasn't a very good one). Bringing another child into that situation would have meant a lot more chaos and struggle for my mum, including the expense of trying to raise two children on her own when they did eventually divorce (as she had no help from my dad). In that case, it really was the best thing that I was an only child and I had a much better, easier, more stable life as a result. I got to go to good schools we couldn't have afforded if I'd had a sibling. We got to travel. I got to do activities and sports I couldn't have done if mum had had to pay for two of us to do it. I never felt lonely or missed having a sibling at all. The only benefit I can see now is that I think it would be nice to have a sibling simply to have a closer connection to my family. I'm not super close to my mum and I have no other blood family in my life besides my daughter, so if I had a sibling (and we got along!), it would be nice to have someone else who was technically family in my life. But I think it's one of those things that you never know how things would have been different if things weren't the way they were. Who knows? You just can't guess these things.
Despite being an only child, we do plan to have two children, but for purely selfish reasons. It's less about them having a sibling as it is us wanting to have two. I'd like the experience of doing it all again now that I feel a lot more confident as a parent than I did the first time. Also, as depressing as it sounds, I had a best friend growing up (who was like a sister to me) who died suddenly from meningitis at 18. She was an only child. So her parents lost their only child, their only chance for grandchildren, the only person who would be responsible for caring for them when they got older. That terrifies me and makes me really sad for them. I don't want to be in that situation and know I'll grow old with no grown children, and as morbid as it sounds, it's definitely filtered into our decision to have more than one.