I'm an only child. When I was a kid I really liked it, especially because all my friends were complaining about their siblings all the time. As an adult I kind of dislike it--I wish I had someone to help me with my parents as they get older. But, as others have pointed out, that's an idealized scenario--my hypothetical siblings could easily live far away, have their own health issues, or just plain be jerks.
Personally, I can't really imagine what it's like to have a sibling. And I've not met many people with close-age siblings, who can imagine what it's like to be an only child, especially growing up. If they dislike their siblings, they sometimes idealize being an only child; if they like them, they often pity only children, and imagine their childhoods were terribly lonely. (And some people of course just admit they can't imagine it at all--don't want to generalize.)
I don't really want to have any kids; but if I did have one, I would try really, really hard to have a second, because I wouldn't want to have an only child--I see too many disadvantages to it. So if someone I was close to had one child, and they were interested in my own personal experiences being an only child, I would be happy to talk to them about it. I hope I would do so in a polite and not pushy way, and not assume my experience was universal. But ultimately I would be discouraging of having only children.
I've seen so many strange things happen in families that I've come to believe it's folly to plan on XYZ happening decades from now--I mean, my grandma had five kids, but only one of them really helps her out on a regular basis, and that's not even the one who lives the closest. (Part of the reason being the way Grandma herself raised them/behaves.) So if she ever thought, "Hey, five kids! People to provide for me in my old age!" she must be disappointed.