Nothing wrong with having an only, it's as legitimate a choice as any other number of children.
It's all about how you handle the situation though.
People fall over themselves on these threads to claim that all the onlies they know are perfect, and this is obviously as far from being true as the stereotype that they're all spoilt yet lonely.
Where we live there are a lot of onlies - we live in a country with a lower birth rate than the UK. I do know some horrendous, spoilt, entitled, precocious in an off-putting way, immature, bratty onlies
I also know some lovely ones and most of them are just kids, not paragons, not horrors, just about as pleasant as most of the children with siblings. For the most part the unpleasant ones have parents who are very over invested in the wrong areas of their children's lives (micromanaging their social lives, or being very pushy and competitive in every area from material possessions to sport to school). The lovely ones have sensible parents for the most part. In other words the same issues as for children with siblings, but over intense parents probably do more harm to onlies just due to not having their intensity diluted! I know my mother is very intense and for all I'm not especially close to my multiple siblings as an adult I'm glad I had them, as despite the fact she always worked being her sole child would have probably driven me to leave home even earlier than I did :o
Nothing wrong with having an only child though, just be reflective about getting the balance between facilitating plenty of social contact for them versus micromanaging their friendships, and between giving them the benefit of having plenty of 1:1 versus smoothering and putting on excessive pressure right - like we all need to.