I think the idea is that many societal choices need to be moved up, not just the decision to have children.

For that to be viable people need to A: not leave academia with 5-15y of debt as a matter of course and B: housing suitable for young families needs to, I don't know, actually exist where the decent jobs are. Until those changes happen kids aren't going to be a popular choice before one (or more likely both) parents have a well established career and significant amounts of money in the bank.