Great topic. Having it in the money matters section is a little limiting, though.
If you look up 'only child' into google, you're going to find all the articles are ardently defending the idea, and almost all are written in your stereotypical sassy list form (10 myths about... 12 truths about). Its way over the top for something that supposedly shouldn't matter much. Thou doth protest too much.
What do you think the implications are?
Extinction, regrettably.
For the kids still growing up right now, I think there's a budding alienation problem that they most likely won't be able to describe to us. The way that I seen it in school, it was the ones with a sibling or two ahead or behind in the grades that never had a bullying problem. Because there was more eyes and ears watching out for them. Ideally they police each other from acting like bastards to other people too. It can do wonders with networking even later in life too, like getting a job. You can abuse that to Brazil extremes, but getting that first dishwashing job for being Tommy's little brother isn't a bad deal.
If that sounds stupid to you, Globohomo's solutions are nannystate anti-bullying snitchfest campaigns and universal basic income, respectively.
You also don't want a society almost entirely made up of old geezers, gents. Its going to sound insensitive to the elderly no matter which way you slice it, but its true nonetheless. I've lived it. Ten to one ratio attending funerals to weddings. Countless retirement center visits to various relatives. And yes, all nursing homes have the same smell no matter where you go.
If you want your aging loved ones to live longer, tell them not to eat junk food, not to take up skateboarding, and not say the n word in an urban neighborhood. But this 'prescribe and procedure' policy on your own folks until they get no less than 95 is a soul crusher, especially when you have to do it several times. Sadly, that's usually where all the money goes at the end of a long life. And hey, maybe avoiding or heavily limiting the amount of kids they had in the prime of life helped them bankroll a lot of personal wealth, but that evaporates instantly if they end up in a nursing home in the last years.