For me, it felt more like this drama was first written with a bunch of people in their late 20s or 30s. And the production decided to add 10 years to everyone last minute. So the childlessness barely registered.
I mean, that friend group was incredibly childish, not to mention the sisters. They got drunk basically everyday, skipped work when they fancied, had little to no responsibility (most still worked for their parents - not that there's anything wrong with working for the family business, but they acted like 15yo watching their mom's shop, not like business quasi-owners ).
Take the younger sister for example, she's supposed to be a mom of a 9yo but she seldom cares for her. In fact it's the other way around. She basically drinks every night... Never is she seen teaching something . Or brushing her hair, feeding her...
Not once are the character seen worrying about money, job security, house payments, the general future, politics and the world... All things that most 40 yo I know do once in a while
And what cemented this is the attitude of the parents/grandparents towards their children. ( Maybe it's a Korean thing, but what kind of parent scolds and slaps their 40 yo child ? or even enforces curfew ? ). That attitude seems acceptable towards college kids, not grown ass adults.
So, I didn't see the childlessness as a reflection of Korea's abysmal birthrate, just as questionable writing. If that was a nod to a broader societal issue, wouldn't the older generation have at least nagged a bit ? Or lamented at the fact ? Wouldn't it have been addressed ?