I don’t have any opinion on ‘only children’ as a group any more than I have an opinion on the population of Manchester as a group. Only children are as diverse and different as children with siblings.
I have an opinion on my own childhood as an only child. I didn’t enjoy it for the most part, particularly as I got older. But that was to do with the way my parents parented. They were very intense and intrusive and deliberately did things that isolated me from my peers. I longed for a sibling to dilute them and their attention - it always felt like two on one.
However, I know many people who have terrible sibling relationships - in some cases abusive ones that have scarred them for life. And I definitely had privileges I wouldn’t have had if I’d had a sibling. It isn’t as simple as only child = miserable, lonely and selfish, multiple children = joy and fuzzy feelings.
I currently only have one child who’s only 3 months old. I think I would like to have two. But I’m very conscious that every child should be wanted by their parents in their own right, so I don’t want to have another just to ‘give my son a sibling’.
Incidentally, I think that the reason only children get labelled and stereotyped so viciously is that (some) parents with more than one child feel a strong need to prove they did the right thing by having more than one - to justify the strain on their finances and relationships and the division of their attention, etc.