I am an only. My dad was one of three, and my mum was one of ten (though only seven of those ten survived into adulthood). My extended family is therefore enormous! I have two DSs of my own. My DH is one of three, and would really like another.
I had a very happy childhood as an only. I had lots of friends and lots of interests. I would do a club or group of some sort every night during term time, and an activity every saturday afternoon too. My parents were essentially taxi services when I was between 7 and 15 to get me and my friends to and from all of these activities. Friends were always welcome round at our house, and my parents went all out in that respect.
I have about 20 cousins and about the same number of second cousins (or whatever it is that you call your cousins' children). There was always a birthday party or celebration going on, and we often did day trips out with cousins, sometimes went on breaks together, and always met up at Christmas. Everyone lived within a 10 mile radious of each other.
I have now moved 250 miles away from my side of the family, though we are close (geographically) to DH's side of the family. I miss my extended family like mad, but that's not the point of my reply.
I had a wonderful childhood. I loved being an only, as I could choose to have limitless company, or choose to have quieter time. I never ever felt that I would have liked a sibling. I still don't. I feel completely secure and happy. Much as I loved my cousins and aunts and uncles, I also really valued my friendships. Whenever I meet new people, they cannot believe that I am an only. I clearly don't fall into the stereotype that they expect.
I would have been very happy having an only of my own too, but DH really really wanted a second child. It took until DS1 was 4yo for us to have another one, and of course, I wouldn't be without DS2 for anything in the world. But I wouldn't have had anotehr if DH hadn't pushed for one. I loved my own family being a family with an only too.
Now, as I am older, and my parents are older, I do wonder what will happen as their health deteriorates, which it inevitably will, even though they are in decent health now. But that's not because I'm an only, I don't think. Its because I live 250 miles away from them and can't move closer to them without DH and I abandonong our careers (our jobs simply don't exist where my parents live).
Bit of an epic post. Sorry. Hope it helps you OP.