I can only answer as an "only" myself.
I went through a phase of wanting a sibling but it didn't last long.
Tbh based on conversations with my friends these days I'm very glad I don't have to deal with some of the sibling issues they had/have.
Siblings doesn't automatically mean they'll get on and be friends (though it's lovely when that happens).
In my social circle very few people seem to have a lovely and positive sibling relationship and quite a few have particularly difficult one.
The "joys" of having a playmate in tap are pretty short lived.
What's most telling I'm finding now is the acrimony of dealing with ageing parents and that in every case the female sibling is expected to shoulder the burden.
I've been asked how I'll support my parents as they age and having no-one to share the burden with.
Frankly (based on experience) if I'd had a brother I'd in all probability be doing it all myself anyway.
If I had a sister we may argue about what's required and create addition stress.
As it is my parents and I have already planned for a number of contingency's because I'm an "only" working mother myself.
As I said, aside from a brief period I didn't want/think about a sibling.
Looking back I had an incredible childhood.
I wasn't at all spoiled materially but frankly being a small family meant my parents had the financial resources to do things we otherwise might not have been able to (holidays/experiences etc).
My parents could also "afford" to be hugely generous with their time. I was the recipient of so much positive focus.
I can't say for sure, but I'd be very surprised if my life could have been in any way improved by having a sibling.
For disclosure I have only had one child. I never wanted more.
I can't class him as an only as he has a half sibling (DH precious relationship).