I think it's often a case of the 'grass is greener'.
I am the eldest of three. Yes, I had the 'instant' playmate with the middle sibling. Though we didn't always get on and she's admitted that she was often jealous of not being 'the eldest' or 'the baby' of the family.
When I became a teenager I resented the 'look after your sisters' if mum and dad went out to the shops or when both of them were at work during the school holidays. Or 'can you take your sister with you' if I went out for a walk by myself on holiday. All I wanted to do was to be a brooding teenager on my own!
Fast forward to a few years ago. Decisions had to be made about my mum's care at one point (dad had died a few years prior). It almost turned to WWIII on a few occasions when one wanted care to go one way and the others wanted it to go in a different direction. Having siblings just made decisions harder to reach sometimes. As a family, we do all get along very well, but that caused a number of difficult and heated conversations that DS will never have to have - he'll have to shoulder the full responsibility though which is the flip side of that coin.
Our DS is an only (started late). The hard bit was making sure he had the chance to make friends when he was younger (he's an older teen now so is more than happy to sort himself out).
The easy bits were that, if we wanted to go away or have a day out we only had one child's needs to sort out. We weren't dealing with a baby and a toddler or a pre teen and teen who both wanted/needed different things.
We only have to get one child to one school. I have friends who have a journey in one direction for one child and then nursery in the opposite direction for the younger one.
We only have one set of school holidays to negotiate. Again some friends have a child in junior school that finished a week later than the one at secondary school this summer. And will have the opposite issue when they go back so their available holiday time as a family is less.
And his best friend's mum now has to ferry her eldest to football practice, get home then ferry the youngest to tennis then go and pick up the eldest twice a week! I'm knackered thinking about it!
And, of course, we have more money to spend on DS - which is just as well as university is looming so every penny is being counted! I don't know how parents with kids a couple of years apart manage that.
So would it have been nicer/easier having a sibling for DS? Absolutely no idea. Any more than I have any idea whether it would have been great or not have been an only child myself. There's no right or wrong, no ideal number of kids. Because every kid is different so every family is different.