I think @Strawberriesandpears is getting a hard time.People experience life differently, some only children will be absolutely fine but some do experience loneliness and that's equally valid.
I'm an only child and recognise a lot of what she says, even though I now have a husband and child of my own. I remember feeling the exact same way, which is why I felt under a lot of pressure to find somebody and start a family.
I also have lots of friends that I see regularly, so between that and my husband/child, work and hobbies, I'm not lonely day to day. The loneliness is on a deeper level.
I have one remaining parent (elderly) and when she's gone (and my aunts/uncles of her generation) I'll have no close family, other than 2 cousins 20 years older who live in another country.
Nobody of my generation to reminisce about childhood memories with, nobody who'll even remember me as a child.
Nobody to call on if I have a crisis, like if my marriage breaks down and I need somewhere to stay, or if someone dies.
Yes, friends will support to some extent but they have their own families and stuff going on, which naturally takes priority.
I'll never have nieces and nephews and my child won't have any close younger relatives from my side. No cousins.
People will say having siblings doesn't guarantee anything and that might be true, but at least there's a possibility that at some point at least they might be close. That option doesn't exist at all of you're an only.
And in my experience, even siblings who don't always get along do support each other when shit hits the fan.
I'm sorry if that's hard for parents of only children to read, I understand why they might react defensively and say it's "melodramatic", especially if having an only wasn't by choice. I'm just sharing how I personally feel.
For me, the cons of being an only go far deeper than the superficial pros like more resources and having been able to do more hobbies. It's hard on an existential level.
I too have struggled to have a second and I'm trying again after a loss. I'll do everything possible to not leave my child an only. If you can afford to have another and are willing to use a donor I'd say go for it.