Wow, don't people love projecting onto others here?! 🤣. As for the 'environmental argument being from people who don't want to sacrifice their big cars and frequent flights', not quite in my experience. I'm a scientist and understand the resource implications. We also don't own a car or fly very often at all. When our only DC arrives I'll be doing my hardest to minimise the environmental impact of adding an additional human to the population.
It's not selfish per se. As my DF always says 'we're all a product of our environment' and a lot of people will make family choices based on their own experiences and circumstances, good or bad.
For those of you wanting viewpoints from adult onlies, I am one and was happy as a child (my parents are also both onlies) and am as an adult. Most of my primary school friends had annoying siblings. I was surrounded by adults as a child (grandparents lived very close to us too) and as a PP has said, I too didn't really like childish messy behaviour at children's parties/dinner tables. I was taught to share with the pet dog (if I got a biscuit I had to get him a biscuit too!) and the adults around me - in the car we took it in turns to pick which cassette we wanted playing. I do have 2nd cousins who get on really well as an example of siblings who have a good relationship, but I totally understand it's not a given!
My DH has a younger brother. Despite them getting on fine as children they now don't. We think it's a one sided sibling rivalry from BIL's side (various events that happened to him seem to be blamed on my DH because my DH didn't have those things to deal with). It's affected our relationship with the ILs too as they went out of their way to put pressure on us to build bridges when we tried really hard to no avail. In particular MIL doesn't seem to want to accept that it's BIL who has the issue! In retrospect I think this is partly due to her own dysfunctional family as eldest of 6 all of whom are pretty self-centred. She had to deal with both her parents' illnesses alone (well, with help from FIL) as was the only one living near them, yet if that wasn't enough they were also obstructive about the best way to deal with it. Despite all this shitty behaviour she tries to keep the peace! FIL was an only and I think that caused him to want two but as always, it hasn't turned out to be the sibling relationship he wanted for his children 🤷