I was a “late baby”. My sisters were 12 and 17 when I was born, and both were out of the house by the time I was 5. So I’m in the unenviable position of technically having siblings, but effectively being an only child.
IMHO, I wouldn’t wish true only-childdom on anyone. You’re always on the spot with your parents; you’re subject to charges of being spoiled, whether you are or not; and you’re less socially developed than your classmates with siblings by the time you start school.
But what’s worst of all is the loneliness. I have no shared memories with my sisters. We didn’t even grow up in the same communities. Any closeness between us was forged after I came of age; when I was a child and a teen, they had no use for me. I went through everything alone, and I’m here to tell you, I would have given up a kidney, or the use of my legs, to have had at least one sibling close to my age and living with me.
However, a lot of people respond to that with “No, you don’t wish you had siblings! My big brother used to knock me down and sit on my head! My little sister wrecked everything I ever owned!” And I know stuff like that does happen. But really: every family is dysfunctional to some degree. Every relationship is dysfunctional. Sure, you have some bad times with your siblings, but aren’t there a lot of good times, too? Given the choice, would you live your life over again with the same siblings (and without changing anything that did happen)? Or do you feel the opposite of how I do: that you wouldn’t wish your siblings on anyone?