Such a sensitive subject.
I am an only child, not sure why, my mum never really discusses why they decided to stop at one, particularly as she had me at 18.
I never felt hugely disadvantaged, but it can be lonely, and I do worry about my parents quite a lot. I would say I am a happy, confident person, but I would have preferred a sibling, if it had been up to me, tbh.
I have two dc, my dd is 3.10yo and my ds is 15mo. I am almost jealous of the bond that they already have, and the knowledge that although they may not get on, they will always have that link to each other. I am undecided as to whether I want any more dc, but I suppose this answers your question: I would never have wanted dd to be an only child.
And on the baby front, dd still doesn't sleep through the night, yet ds has slept through since he was 9 weeks old. It wasn't easy, I'm not a fan of the baby stage, but it really is such a short period of time.
Do whatever feels right for you and your family, everyone brings their own agenda to this subject, and it will just confuse you even more, imho.