Hi fortunate
I'm in exactly the same position as you.
I seem very gregarious but I'm in fact an introvert - forced socialising stresses me out and before DD, we SO rarely entertained, as DH is the same - and I do worry that I'm going to have to get over this in a major way to facilitate DD's social life if she stays an only (as I think she will)
Playdates are super-stressful for me right now and I find it particularly hard having people over to us (I'm much better when we meet people at theirs) so though I love the notion of DD always having the house 'filled with friends' the way so many people seem to be able to, and the way that seems ideal for an only, I don't know if we'll ever be in a position to do this for DD!
Maybe as they get older and make their own friends this gets easier as it's less about socialising with the parents too, I don't know.
DD is a very sociable little thing at the moment (though she does enjoy her own space too) and makes friends everywhere we go, so I think this is something I'm going to have to find a way to facilitate for her. After all, I'm the one making the decision not to have another child, so I don't think she should suffer because I'm happy staying on my own without people around all the time!!
That said, you're right, I don't think siblings always get along by any means. I can think of several of DD's little friends who have an older sibling (usually a brother, in this case) with whom they just have absolutely nothing in common and don't play together at all. Let's face it, the average, say, 6 year old boy is just not interested in the same things as your average 3 year old girl. So I think you'd still be forced to arrange social things for them even if they did have a sibling.
We entertain DD a LOT, but that's just us - it works for us as it suits our personalities. Happy to muck around and be silly, but that's not for everyone and nor should it be!
But yes, I do suspect that having an only means you do need to facilitate socialising for them IF that's what they want. I do need to get better at this but it fills me with dread tbh.