I just don't think there is any point thinking about this. I don't mean that horribly.
Your only child could resent you forever for not giving them a sibling
Or they could be perfectly happy as an only and not give a shit
Your child with a sibling could hate their sibling and wish they'd been an only
Or they could have a wonderful sibling relationship and not be able to imagine life without them.
You can't know. You can't predict. You can't plan. All you can do is make the best choices for your own family, using the information you have in the here and now.
Personally, I'm one of four and I can't imagine life without my siblings, even though I'm very introverted and spend a lot of time on my own. However, my DS is nearly 4 and I'm only just coming round to the idea of having a second. Reason being that I didn't want another child, and no way was I bringing a baby into the world just so ds would have a sibling. It's not a pet!! I now want another baby, so I'll consider having another.