I don't disagree with your point that the article is overly simplistic with relationships (presumably because it is told from the POV of a person who is unmarried with no kids) but it also is more complicated than just assuming everyone will get married, stay married and have kids.
Your point is well taken, but divorce rates are dropping, so while we aren't living in the world of "Leave it to Beaver", neither are we in the lousy marriage climate of the 80s.
The other comment was closer to the reasons I've read - later marriages, and/or people who do not marry in the first place.
I am 64 and in my experience interpersonal relationships keep getting better with age.
Zero understanding? What's there to understand? The article isn't set to examine the quality of human relationships, or their intricasies, just their quantity over time.
It's also obvious, at least for spousal relationships, that those wont slow down over time, you'll continue to see your spouse everyday -- so it's clearly a relation outside of the scope of the article, which is relations were "time together" diminishes with age.
As for the children, while you'll continue seeing them everyday for a couple of decades after having them, after they're around 18 you'll have seen the most of them you'll ever will, even if you're only 40 and have 40+ years ahead of you. And that's for the same reasons mentioned in the article. Especially if they move to another town/state, but even if not.
It does use the word "best" in the title.