I was originally planning to have only one, then changed my mind at the last minute and had a second with rather a large gap, which in some ways can be a bit like "two onlies." So I have sort of seen both sides of this.
The only-child families I know definitely do have it easier in many ways. It's easier for both parents to work full time and pursue careers, there is a lot less expense - unless you choose to spend more on your one child - you will have more money for private schooling and/or expensive extra curriculars if you want! Travel is also easier - both logistically and financially. The dynamic of one-child families is very nice in most cases, and you don't get all the tiresome sibling fighting; only children tend to orient more towards the parents' interests and the family culture has a more "adult" feel. There is also no evidence that being an only child has any negative impact on children's development; there is no evidence they are more likely than sibset members to be spoiled, selfish or poor at interacting with others. Onlies may have slightly different dynamics with their parents, but it does not seem to result in different behaviors with other people outside the family.
What worried me, when I was assuming I would only have one child, was the existential and long-term picture. There is a certain feeling of having all your eggs in one basket. I think most people do accept, honestly, that the death of a child is worse when they are an only child. Not because a sibling is somehow supposed to "replace" the deceased child (no human can ever actually replace another), but because bereaved parents who still have a living child will still have their identity as parents and something to motivate them to pick themselves up and go on living and getting joy out of life. If I lost an only child, I can't imagine how I would even begin to recover it and what I'd go on living for, honestly.
And the "aging parents" issue is a real concern. I know that the standard retort to this is "Well, parents shouldn't expect their kids to look after them in old age!" I totally agree, they shouldn't. But the reality is that decent people do generally end up feeling obliged to ensure parents are not left alone, regardless of how much the parents themselves urge them to think about themselves and reassure them that they have no responsibility. The adult only children that I know all feel the difficult "pull" of having aged parents who they don't want to feel abandoned, and they feel this struggle despite the parents themselves never having put any pressure on the kids whatsoever to feel this way.
We are an international couple and there is a fair chance that at least one of our kids will end up wanting to settle overseas, because they are bilingual and people who were born into international marriages are much more likely to enter such relationships themselves IME. It's easier if there is at least two siblings, as in all likelihood one will choose overseas and the other will not - at least, my experience of sibsets born into international marriages is that this is generally the way things tend to go! With an only child, I would hate to feel that I had put them in a situation where they felt guilty about wanting to make a life overseas, or worrying that they felt pressure to settle in the country where we live despite perhaps secretly wanting to live elsewhere.
Those were among the reasons why we ultimately chose two children, and are OK with the extra hard work involved. I'm hoping to grow old with more than one stake in the future, and that our kids will be glad to have each other in the long term. That said, I would never ever pressure anyone else to have an additional child or ask loaded questions like "So, when you are having your next one?" or whatever.