Peoples perception is that you can’t share, you’re going to be weird and spoilt and have no social skills.
I had lots of cousins my age and I didn’t really feel lonely, though I do think I missed some social queues and I was a bit envious that my cousins had a different bond, I recognise that my parents made a real effort to make sure I played with lots of children, had lots of sleepovers etc
Only talking to adults meant I was very bright and mature for my age and that served me quite well. As a kid I was happy that I could play with my cousins and friends and then leave and go home to my quiet home. I never wanted a sibling and was glad I didn’t have one.
As a teenager I always wished there was someone around to shift the focus a bit, and minor indiscretions seemed huge but still I didn’t really want a sibling.
As an adult I am sad I don’t have the bond some of my friends have with their siblings and I’m aware my parents are completely my responsibility
Equally I’m sad for my own children that they won’t have any aunts uncles and cousins on my side.
I do think I am quite self sufficient which is great but also means sometimes it’s hard to make good friends as any time there’s drama I am happy to walk away. I had lots of time and energy (academically, psychologically, time etc) focused on me which has helped me progress in life.
I don’t think it’s a day to day problem and you have to do what’s right for you, if you can’t cope with more DC then that will have different negative effects.
I personally wouldn’t have an only child, but in part that is because I’m aware they would be an only child with no close relatives like cousins and aunts to make up the difference.