Full disclosure, I'm an only child but did not choose to have an only.
With time and maturity I've come to see there are massive benefits to being an only, but also some drawbacks. Those benefits and drawbacks I think are more heightened than other family sizes. Here's my 2 cents.
Most families with only children have a calmer family life, as mentioned by several posters. Of course there are exceptions, but on the whole it's true. There aren't sibling squabbles, only children are more adapt at entertaining themselves and benefit enormously maturity wise from more adult environments. While I didn't choose to have an only, we have close friends and family that do and like my experience their children ranging in ages from 5-11 are well adjusted, intelligent, gentle and well rounded. They have some wonderful added benefits including for some private school education, devoted parent and grandparents time/attention and certainly don't need any to pity them. The only other families I've seen with this same sort of benefit/dynamic are families with large age gap and only 2 children where in a way there's a sibling but due to large age gap the needs are very different. For example siblings that are 7+ years apart. There are studies about the cognitive benefits of being an only child and I think they hold true.
The drawbacks of being an only child are that you can end up isolated and no amount of friendships may change that. For example, my three closest girlfriends all have 2 sisters they are very close to. Once they were past the annoying sibling part of life, they developed very deep bonds and have chosen to live near one another, holiday together as families, have weekends away and be one another's closest supports. Of course that doesn't hold true for all siblings, and there are terrible siblings out there whose sibling relationships probably bring more strife than happiness. But with a sibling you have a chance of the longest relationship of your life, someone to share in your joys and hardships, someone who you can support and who can support you. Most siblings identify with having a good relationship at least and that's different to friendships. Friendships can be far more transient and you are far more likely to lose touch with a friend than a sibling. Again it doesn't mean that doesn't happen, but statistically. In addition as parents age and die, a sibling gives you that link to the family you were born into and raised by. Without a sibling that is gone.
A lot of people also say their only children can get their role as aunt/uncle/sibling from the family they marry into with SIL's, BI's and nieces and nephews. But you can't guarantee that. They may marry into a family where there's been early death, or another only, or the adult sibling of their partner has SEN. One of my friends who has an only always planned to and ended up marrying into a family where he'd already lost both his parents by age 20, as well as a sibling. He has one sibling who has SEN and is more like a toddler. Their child has no aunts/uncles/cousins and one set of elderly grandparents. Another friend never met anyone and had a child on their own. Their child has 2 family members - a parent and one surviving grandparent who likely won't live another 5 years. Finally another with an only assumed their siblings would raise their children as close to their only who arrived after a decade of infertility. Instead because their siblings each had 3 and ended up living near inlaws who have multiple adult children with lots of cousins, while my friend is close to her two siblings and they and speak weekly, their daughter has not had the benefit of close cousins. They live about a 5 hour drive apart and see each other no more than twice yearly for a day.
I don't judge anyone for having an only, nor should anyone have a child begrudgingly for the child they already have. Only children families are just as much a family as families with more dc. I can't stand the idea that it is the second child that makes you a proper family. I'd go as far as to say that many families would be better off in terms of not only money but also capacity, mental health and relationships if they only had one. But it's never a surprise to me that only children are more likely to have larger families than people who are raised as one of two or three kids.
For me I always knew if I had kids the minimum would be three. I have no doubt that was shaped by my experience of being an only child. An experience that brought lots of privileges but also a loss of something that neither money nor time could buy.
Good luck!