For those following at home, Bethany has recently written about birthrate as well, with different focus. It makes the comments sections a little unwieldy, so I am adding my new thought here as a separate post, rather than updating the first one or following on to T99's comment.
Children are lots of fun, but they are a lot of work. I think you knew that. One of the things that softens that is being an aunt or uncle. I have heard women in particular, if they feel they are being judged for having no children, mention that they are very close to their nieces. Though keeping it to myself, I have been a bit dismissive about that, because it's not the same. On the other hand, it's got similarities, and as I have watched my three boys with no children interact with the daughters of the two who do, I can see the value for both sides more clearly than I did when my children were small. The uh, quality of the players may have something to do with that. One has to put in some effort* to be a good aunt or uncle, but one gets considerable reward in return.
But if birthrates are falling precipitously, the supply of uncles and aunts dries up in a generation. The last two generations have likely hit the sweet spot, with more attention from Mom and Dad (smaller families) plus more aunts and uncles to swoop in at times. Or at least, it would have been the sweet spot if Moms and Dads had stayed together. Holiday gatherings and family reunions get complicated, and less frequent. I suppose the great mobility of the last two generations has undermined the closeness to nieces and nephews as well. My nieces would wish openly for cousins, and were ecstatic to get some. (That they were girls, even better!)
Italy, Portugal, Japan, Greece, Spain, South Korea...these all have very low fertility rates. In the first generation, a child has no siblings, or perhaps one. But when that child has children there may be only a single aunt between the two sides, and 0-1 cousins. For Americans used to large Italian-American families, the idea of an Italian culture with no aunts or uncles, no cousins is inconceivable. Yet that is the current reality. It's not getting better.
If you thought the disappearance of the middle child was a big cultural difference going forward, wait until you see a world with no cousins or uncles, where even the concept has something of a last-century feel to it. The word nepot is 6,000 years old, and descends unchanged from Proto-Indo-European to Romanian (and you can see the root clearly in many other languages). It is still useful, but what will be its use in even fifty years?
I have cousins on one side, and even knew half-a-dozen second cousins. As with siblings, that is not an unmixed blessing. Yet for grounding one's memories, or getting the other side of controversial family happenings, or just being nostalgic and reassuring oneself that one had a beginning. Those with close or many siblings have less need, and cousins separated by age or distance don't always provide much added benefit. I was close to one uncle and one aunt - I have been close to two nephews and moderately so to three nieces and another nephew. The benefit is real.
*If you are the first in your family to have children, then you get to palm some of the effort off on your own children, who love being the oldest cousins.