Well it started out as a nice thread....
I have one dc9 and I love parenting her. It's been an absolute joy so far. I count my blessings when I see stressed parents at the school gates with multiple kids crying and hanging off their leg whilst the teacher is trying to speak to them.
My DC is so loving, polite, funny and sociable. I always thought I would have more but couldn't. It has actually turned out so well and couldn't recommend it more!. We all have a great life. Dh and I have lots of hobbies, great social lives , weekends away and plenty of willing babysitters. Its much easier getting a babysitter for 1, and when dc has sleepovers we also get a night off (we obviously reciprocate).
Dc has many many friends and always has friends over/ goes to theirs. Every parent comments on their lovely manners. Never had an issue with sharing, taking turns etc, if anything they are overly generous (we're working on it!.). Dc went to many baby groups, then more structured activities as they got older.
I especially love being able to snuggle on the sofa and watch a movie in peace. No fighting over who gets to sit next to mummy!
I am one of 6 and whilst I have a good relationship with siblings I would say my dc has a better life. I have a best friend from school who I see more than my siblings so no need to adult siblings to fill that role. Compared to my childhood, my dc has more time with parents, stronger bond, friends that can come around a lot (which they always do- they prefer my house as no crying, moaning younger siblings to contend with- their words). Dc also does a sport competitively several times a week. I don't know how this would be feasible logistically if they had siblings who had their own activities. I had to give up a sport as my parents couldn't juggle the Saturday competitions with other dc's needs. I still think about how my life could've been different if I'd followed my passion.
DC is never lonely (we talk very openly). She has cousins similar in age so has friends to go on holiday with, day trips etc.
The above is only my experience obviously, but in the end I do think that parenting style and general environment are more important factors in quality of life/ happiness etc than whether or not one has a sibling.