I'll add my own conjecture: the root problem is that this hypothetical couple you describe (who is more common than many would like to admit) are both settling.
They are settling because they didn't take marriage seriously earlier in their lives. They didn't take marriage seriously because society stopped forcing them to do so. And nature also didn't force them to either, thanks to birth control.
These aren't individuals at fault here - these are systemic incentives warping previously traditional behavior.
So then this couple got into their 30s, which isn't young, as the decades are not created equal. And they realized - oh shit, better actually try figuring out how to have a family, lest we live alone and with limited meaning for the latter 40 years of life. Seriously - what are people thinking that they'll be happy being single into their 40s and beyond?
Here's a sad truth: most people will get a decent, but not super meaningful job. There can only be so many entrepreneurs and rockstars. So from ages 35 onwards, you better be set up to have a family and create something that will provide you meaning for Act II of life, because it's quite long.