I grew up as an only (I was my mum’s only child but not my dad’s, but my half siblings are a generation older than me). I certainly wasn’t spoilt financially as my parents were never in a position to do that, but I did tend to get my own way quite a lot (not all the time). I can remember wanting a sibling when I was quite little but it was never an issue later. I definitely did have to learn to share in adulthood - in student shared houses, to be exact, and it was a baptism of fire. Hated that! I was always a terrible housemate, of the ‘Who moved my mug?’ variety. 
It’s still something I struggle with tbh. On the other hand I’m very untidy if there’s nobody else there to see it. I think that’s more to do with my mother’s overindulgent parenting rather than being an only child specifically - although I guess she might have behaved differently if there had been more than one child. No way of knowing though.
These are the downsides. But: I’ve always been happy in my own company and don’t really understand loneliness - people talking about being lonely during lockdown is something I get on a rational level, and I can make the right noises when they say it, but emotionally I can’t relate to it at all. The last 18 months hasn’t bothered me, in that respect at least. Both my parents died relatively young after short illnesses, so I was lucky to avoid long-term caring duties, but god was I happy to have sole responsibility for the house, wills etc! (And frankly, very happy to receive the whole - albeit small - inheritance.) When I see the disputes that people get into over parental estates - including my half-siblings, over their mother’s house - I think, ‘No thanks, not for me!’
The only thing now is that with both parents gone, I do occasionally wish I had someone else who shares my memories or who could fill in the odd gap, but it’s not something I think about often. I have a cousin (also an only child, coincidentally) who sometimes messages me to reminisce about our shared grandmother and other family stuff, and old friends of my mum’s who are still alive, so that’s something.
My DS is also an only child, by accident rather than design. I always said I’d have more than one child but it just didn’t happen due to circumstances. Deep down though I’m quite relieved. I don’t think I’d have known how to raise siblings! I did make a point of a) socialising him with other kids extremely early (something my parents didn’t do as we lived in a remote area until I was 3), b) teaching him explicitly and repeatedly about sharing, and c) trying not to let him behave in an entitled way generally.
It’s hard though and I’ve caught myself repeating my mother’s mistakes more than once. All things considered, I’d say he’s a more well-rounded, sociable kid than I was at his age, although the lessons of early childhood are fading as he moves towards his teens. I do worry about him having to care for me or ExH later on, but that’s to do with our age. If I had two kids then I’d just worry about him having to care for us AND think about a younger sibling. He does like to be pampered a bit, but at his core he’s very tough and resilient, so I think he’ll be ok.