I am an only. It was a choice because mother didn't want children and I grew up knowing it. Also, I was born in 1960 and it was remarked upon and there were negative biases.
Mother, and father, are/were also onlies. Mother because that's what nature delivered, father because his sister and parents did not survive the holocaust. Mother was utterly adored and also very spoilt; I was not.
My MIL is very free with aggy comments about my onliness and how onlys are selfish and spoilt and need attention. Paradoxically her own dd's are greedy and opinionated and one has a significant aversion to work.
I always loved work and always played it well. I may have been a bit of an island because I'm an only or it moght be because I am professional and don't over share at work.
The negative comments made me hate being an only. That may have been more felt because I wasnthe only child of two onlies and I worroed when Inwas yiung and before I had my own family that I may may be all alone. That was why I did not want an only child but two or three - we were dealt two and it was a battle to get them.
With age, I see that despite DH's cousins and aunts and uncles and two sisters, when the chips are down ot's friends who matter and dh is his mother's sole carer because hos sisters live abroad and frankly do fuck all. So with old and aging mothers, DH and I have equal responsibilities. However, I will get all the money, whilst DH will shsre it three ways despite his sisters doing fuck all, rarely phoning and more rarely visiting.
Nowadays @ConnectThree I think an only has become unremarkable.
Having said all of the above, however, our two are very close and I think will always look out for each other but many of their frends are not close to their sibs.
Ultimately, if an only was/is wanted, I think it's fine. They will likely be more independent and forge good friendships.