I think the only child argument is nonsense. A child should only have a sibling if the parents want more than one child. It's easy enough to encourage friendships or relationships with cousins etc. Only children benefit from undivided parental attention too, and all day trips holidays etc can be planned with their needs in mind.
Ours are adopted. We had our first for almost 6 years before the second arrived. We really wanted a second but ended up waiting a long while for a placement. While we have never regretted having two, the second placement had a major impact on our one-child family lifestyle that took a lot of adapting to!
Also they have never really got on - total opposites in almost every way and as they got older, seemed to deliberately not agree with each other's choices - activities, films, food choices etc. Shopping with both but only one adult was something I avoided when at all possible.
Obviously adopted kids from different families are less likely to be similar than most children, but one of my family members, for example, loathes their sibling and vice versa, to the extent that when the sibling was unwell with a possibly fatal illness, they refused to visit them.
Personally I have two siblings who mean a great deal to me, but it doesn't work like that for everyone.
I do understand the yearning for another child though. For a long time it seemed as if we wouldnt get a second and I felt our family would be forever incomplete. At marriage I would have wanted 3 but the complexities of adoption and being that much older by the time we got round to going through the process, made that seem definitely too much for us.