I find all of this talk around only children totally baffling. If you want to go into hard data, science says over and over that only children end up no different than first born children in life outcomes, and that includes reported happiness, success, loneliness, etc.
The two-child model in the West is completely cultural. The reason we so deeply value having two kids in Britain simply comes down to what is culturally trendy. At the turn of the last century, we actually valued having three or more kids, and families were often mildly stigmatized for only having two children! Then two became the model of choice, so having an only child (or three!) was stigmatized until very recently.
The sibling preference is really, truly cultural. After China lifted the one-child rule and allowed families to have as many kids as they want, most Chinese families still opted to only have one child, because that had become the new ideal number. People thought, why on earth would you have two kids when one is the ideal number? So in other countries, having siblings is undesirable and one-child families are the best kind of family! It's still considered the easiest, simplest, cheapest, and most rewarding family model in China.
And of you're curious, the current trend for wealthy Britains is actually gently moving towards having three children, not just two! Why, you ask? It's a quiet display of wealth. Property prices and cost of living are rising at the expense of stagnating working wages - at a time when it costs the climate and burns a great deal of expensive energy to have more children - so supporting three children is a huge way to show off your wealth and privilege (especially if you have a nice house, lots of nannies, and send them to nice schools... like William and Kate! Always count on royals to lead the way on trends).
So having a sibling for your firstborn kid is purely a cultural and personal choice. There are no more social downsides for the only child than if that same kid grows up in a family with other kids. That's the reality. A reality backed up by good scientific and cultural data.
I had a sibling and I can tell you that I was lonely growing up and needed playmates as much as anyone else. A sibling didn't magically make me less lonely. As my parents have aged, the burden has fallen on only one of us siblings because of where we live, so the benefit of having two kids didn't really help one of us as our parents aged. In our case, having a sibling has just added resentment and complication. Also, my brother is a lovely person, but we are so different as people, we don't talk in adulthood in any deep or meaningful way. My friends are much closer to me. This just adds this extra layer of guilt, like my brother and I feel like we should be close, even though we have nothing in common. Frankly, if siblings are close to each other, it's the luck of the draw. Parents can't force a relationship. There's only so much a parent can do to make siblings like each other. My parents tried very hard for my brother and I to be close, but honestly the best we can do is be friendly acquaintances. Which is okay. But not any kind of big reason to have a second child.
I always think the grass is greener. If you grew up as an only and were lonely as a child, it's easy to blame the lack of a sibling. But the reality is a lonely childhood would have been a lonely childhood. A sibling probably wouldn't have changed that. If you think it's hard to take care of aging parents by yourself, the reality is you would probably still have had the burden but with added anger and resentment towards any siblings who don't help as much. I can imagine that only children may feel a sense of loss when their parents pass away, but they would have been equally likely to have had massive falling out with siblings in the aftermath of inheritance or hate siblings so much they never wanted to spend time with them. Again, luck of the draw.
All of this is to say... there's absolutely no downside to being an only or an upside to being a sibling outside the immediate family and life situation. If you want one, cool. If you want two, cool. If you want eight, cool. Whatever. It's a personal choice.
Two or three is not some magic number. It's just the current cultural trend. Siblings aren't the greatest or the worst thing. They're just a thing.
Decide what you want for your family. Don't tell other families what is best for them based on your pure feelings, because your feelings are probably rooted in social expectations. And what society thinks is best right now is totally a passing cultural trend. So make this decision for you and no one else.
(This message was brought to you by your friendly neighbourhood anthropologist).