It's not that they can't thrive in a pinch and more "what do you do when it's normal". Let's also remember that the person can have multiple kids in a row. A long time ago I worked at a company where the VP of a department ended up with 3 kids back to back. By the time they came back it was basically a different company and no one remembered them. The company couldn't tell ahead of time if this was going to keep going or not, so they had a new permanent VP, and no real room for an extra one. They basically split up the department in 2 for no real reason since they had to give the person a similar job back. It was really weird.

Obviously those are extreme case. Companies are dealing with it all the time now, and it's pretty normal. They make it work.

But there's no real argument that it leads to some really awkward situations sometimes.